The eight hour flight from New York to Seattle...

A couple of reasons, but this answer will be based on the assumption that while medical attention was needed, it was perhaps not a critical emergency.

The distance between Seattle and Anchorage is less than 200 miles, so if it was not a medical emergency that required immediate landing, then doing so in Seattle would be better because ANA has contracts with ground suppliers, and thus have resources available at Seattle and none at Anchorage. And while Vancouver is marginally closer than Seattle, again if it's not a medical emergency, it makes more sense to do land in Seattle because there are less complications related to immigration, and that ground crew resources are available in Seattle. Remember, ANA also needs to take into consideration of other passengers whom are affected by this landing, and thus, they need to ensure that the airport also has the necessary resources to handle these passengers (who may need to be re-booked, re-accommodated, etc.).

Seattle is close to Boeing if they needed parts?

I think Adrian pretty much has the reason - existing infrastructure. YVR/SEA are basically the same distance-wise, and I'm guessing ANC would've been a worse location.

Since the flight originated in NYC, I imagine immigration would be less complicated in SEA than in Vancouver.

Everyone else said it, mostly. Immigration and ground services already in place. But I'd also mention that if they had to cancel the flight because the crew was too tired (delay for 24 hours or something), they could move a number of the passengers (depending on available capacity) onto the Seattle flight.

Immigration and ground services, like everyone has mentioned. Unlikely in this situation, but there's also cargo considerations (e.g. hazardous or sensitive materials) and the regulatory implications for bringing that into another country.

The 777 and 787 have very comparable flight decks. Boeing designed the 787 to reduce the need to retrain pilots for a completely new aircraft.

Why not Edmonton??? Yellowknife?

There's also the fact that it's getting to be the cold season in Anchorage, so maybe temperature and/or related resources could've been limiting factors.

If indeed it was a medical emergency, there are some type of emergencies that are much better dealt with in Seattle then Anchorage. Seattle is the referral center for Alaska anyway and depending on the nature of the emergency it might not be able to dealt with appropriately in Anchorage. If I had to make the call the the distance was equal, it would always be Seattle just based on the availability of advanced care.

I assume JFK-NRT is a popular flight for ANA so that flight might have a full load of passengers, cargo, and fuel to make it to NRT. Being over Canada and no open body of water to dump fuel, I'm suspecting the crew decided to burn off the fuel by extending the flight a couple hours versus landing at the next available airstrip to reduce the weight rather than asking Canada for permission for an overland fuel dump and the risk of fines. This helps to prevent the aircraft from landing in an overweight condition and possibly burning brakes and blowing tires which requires a detailed and time-consuming inspection of the main gears. If it did, then SEA is the perfect place due to ANA's already available contracted maintenance and a convenient supply of spare parts up the road in Everett in case something did break to make for a quicker turnaround versus an air interruption into a place like Edmonton or Yellowknife. Or I could also be completely wrong and it was actually immigration.

The legalities with an unscheduled stop in Canada would be nuts, especially if passengers had to stay over night. Canada is super picky about who they let in the country (we almost didn't let Martha Stewart in once because of her criminal record) so I'm sure that was a consideration as others have mentioned. But still. that would such for those passengers.

I was surprised to learn that NH doesn't have service to YVR.

Do they then reposition this plane from HND to NRT?

NH 1009 has a lot of Asian connecting passengers going beyond Tokyo. Landing in Vancouver will likely cause visa difficulties for Chinese/Pilipino/Thai/Indonesian and some other nationals as they won't be allowed to leave the aircraft without a Canadian visa.

I think this choice might have been due to the availability of the next crew. The few reasons people have mentioned don't really make that much sense to me. First with the fuel dumping/burning issue, if they needed to burn fuel, it would have been much better to simply stay in a holding pattern near the alternate airport to do so, this way if the situation suddenly takes a turn for the worse, they can get on the ground quickly.

As far as ANC vs SEA, as far as ground handling, ANA uses Swissport at ANC for their cargo operation, all it would have taken was a simply phone call and Swissport would have easily have arrange a crew to handle the passenger jet since they are the biggest ground handling company at ANC. And Anchorage is a large city, hotels are certainly not going to be a problem.

I think the main reason for SEA is simply because the problem of getting a crew to ANC. Any crew already there from other flights would likely only have 747 type ratings, and there would certainly not be any flight attendants. It is very likely that there's already a crew on the stop at SEA, and since 777 and 787 shares a type rating, it would have been a simple matter of doing some crew schedule changes, they might not even have to bring anyone out there.

Also remember When Ana started service to Seattle it was on a 777. The crew and pilots in Seattle are probably crossed trained

Everything already said ...
Need 3 hours flight time to burn the fuel down to max landing weight (you can land above it, but the aircraft will need inspections), better re-accom potential and prefer USA for immigrations.

Another data point ref immigrations:
I used to work Ops for AA at DFW (so I could give you many stories). One day our DFW-MAD 767 returned after about 15 minutes with a mechanical. Maintenance felt they'd only need about 10-15 minutes in the cockpit to clear the item. We had already filled all the widebody Intl gates so called Customs to let them know we were going to park the flight on a domestic gate and asked if they wanted a Customs Agent on hand. The answer was that we had to park on an Intl gate or be fined for every soul on board. That flight always had a lot of TRWOVs onboard (Transients without a Visa) as it was a popular flt for Central America passengers going to Madrid. These passengers had not cleard USA Customs for their connection at DFW. As the USA passengers had now had the chance to "mingle" with the TRWOVs, Customs declared all passengers as having departed the USA, even though the flight hadn't even taken off yet !

If ANA doesn't offer "transient" boarding at JFK (all passengers were "in" the USA when boarded), then I'd bet Customs would have no problem allowing the flight to re-enter the USA as a domestic arrival.

I have a similar story to offer. In July I took UA SFO-HKG. After flying for 4-5 hrs, the pilot announced that due to medical emergency, we needed to fly an extra hr to ANC. De-boarding that passenger and some sort of inspections took 2 hrs. When we all thought we could continue, the pilot announced there's not enough hotel rooms to accommodate the whole plane, and hence we're flying to *drum roll* SFO!? Yes, after 11+ hrs leaving the airport we returned to SFO. Only the premium cabin passengers were given hotel rooms (reason was "not enough hotel rooms"), the rest had to crash in the airport at 2am. We were never given an explanation why SFO and not SEA. Later I found out from the crew that there's union issues involved, and after reading the comments, maybe the fuel was one of the factors too...?

@lamb
It makes sense that if it were a 747 they would return to SFO, because that is where most of the 747 pilots are based, so they would have a much easier time getting a new crew. SEA just has one 777 departure a day so they would have to fly in a new crew to continue the flight.

Maybe I'm just reading too much into it, but it says "injured" not "ill" passenger. Was the injury sustained in flight? I mean, someone already seriously injured pre-flight wouldn't get on (or be let on) an int'l commercial flight, I would think.

this is what happens when old people try to join the mile high club

@ Claire -- I'd be willing to bet it's just a bad translation. Wouldn't be the first time in Japan...

If it were a true medical emergency they would have diverted to the closest suitable airport instead of flying down to SEA. In such a situation, getting the passenger on the ground takes priority over almost anything.

Based on their position, Edmonton would have been the nearest major centre. It has an international airport with nice long runways and medical facilities as good as you'll find anywhere.

The fact that the flight went to SEA instead of diverting to the nearest airport implies the issue was not that serious.

@Adrian, the distance between SEA and ANC is over 2000miles...not 200 miles.

Did you mean Vancouver?