Air Premia Launching Seoul To Newark Flights

35" legroom in Economy is more than some so-called Premium Economy seats I've seen promoted. Impressive!

They are flying to EWR because there is a huge Korean population there that goes back and forth. And while I assume they use JFK because that's the only option, this would be better and closer for them.

Newark was a gaping hole in service from the US to Seoul. It was waiting for someone to start it and the chances are it was not going to be United.

There's a very large Korean population in Northern New Jersey. Air Premia sees a route that is ripe for the picking.

They're actually already making profit from their flights. Business class demand is very low from South Korea because Korean companies never let their employees travel in business class unless they're execs. Even if it's Samsung or Hyundai, they never do.
Also, Koreans simply don't want to spend much on flights. They rather spend more on hotels and food, not on any sort of transportation. The only korean carrier that makes meaningful amount of profit from business class is Korean Air only, because they dominate almost all the business travel demand(of execs and government officials) out of Korea. Even Asiana can't make much profit from business class.
That's why Air Premia decided to operate without business class. Only Korean Air can make profitable business class, not others. Instead, Air Premia wants to steal economy class customers from Korean Air, and it's working very well. Air Premia is cheaper and even has wider seats than Korean Air, and they also have in-flight wi-fi that Korean Air doesn't.
Air Premia is more like a FSC without lie-flat business class. They offer free meals, beverages(alcoholic drinks are paid in economy class though) and free checked luggage.
How do they make profit? Because they have more seats than Korean Air. Korean Air has 24J 245Y on B789 whereas Air Premia has 56PY 253Y.
And Air Premia is filling over 90% of the seats on LAX route without any award seat. They have "revenue flights" only, no award.

They could have put over 330 seats on B789 if the seat pitch were 30"-31", however, they wouldn't have been able to make that high load factor with uncomfortable seats. They made it 35" to steal economy passengers from Korean Air which offers 33"-34" pitch. And the saved weight can be allocated to the cargo.

Why don't they fly Business Class on other airlines?

I've notice KE filling up business class seats too. They seem to fully load their A380 lately.

There are three reasons:

1. Network
Not any airline can beat Korean Air's route network out of Korea.

2. Reliability
Korean Air is one of the most reliable airlines in the world. Even if a flight's empty, they don't cancel it as long as there's any passenger booked. Reliability comes first when Koreans book their business travel. Korean Air's the best option to guarantee their travel plans.
Korean Air doesn't have any fancy features, nor any inflight wi-fi, but business travelers fly them only for reliability.
Koreans never accept any uncertainty when it comes to business travel. Even if it's the airline's fault, the employee will be castigated by their employer when there's any disruption on his/her business travel.

3. FFP
A lot of Korean business travelers(mostly execs) are tied to Korean Air's Skypass program, so Korean Air is always the first option for them, and the second are Skyteam airlines such as Delta, Air France, etc. Also, even when they fly other Skyteam airlines, they prefer Korean Air codeshare so that they can get elite qualifying miles and that Korean Air can offer alternative flight options in case of flight disruptions.

Air France and Delta also makes meaningful amount of business class passengers to/from Korea, but non-Skyteam airlines can't. This is why British Airways terminated its service to Seoul. Even JAL has codeshare with Korean Air on its GMP-HND route to survive.
Star Alliance airlines could also survive in Korean market thanks to Asiana, but since it's gonna be merged to Korean Air, it's a matter of time

Oh and I forgot to mention this..

4. Flexibility
It's illegal to sell non-refundable/non-changeable flight ticket in Korea, and this "must-be-flexible" rule applies to Korean carriers only.
Therefore, all the tickets from Korean carriers are flexible albeit with some fees, and even the fees are set by the government, not by the airline.
It's very common for Korean companies to change their travel plans, so the flexibility of Korean carriers is a huge advantage for them. Not any non-Korean carrier can offer this level of flexibility at this price. Even the cheapest economy class ticket from any Korean carrier is flexible.
It costs a lot more to fly non-Korean carriers with this level of flexibility, and Korean Air offers all the aforementioned advantages as well.

More competition is good for customers. Air Premia seems to have a great economy product.