We are not starting Newark service as planned either, short crew there too ?
I would suggest it's a slot problem. Their desire to get even more NY flights suggests that there is money to be made in that market, so logically, they would probably slide the shortage onto other sectors, by canceling/combining regional flights as is already happening (eg. TPE, BKK, MNL).
Flying to a different airport in the same city is not the preferred option, as there needs to be a whole new set of contracts put in place - hotels, refueling, ground handling etc etc. Implies that there is enough money to be made that if they can't get another slot at the moment at JFK, it's financially viable to start a EWR service despite the extras.