EWR/Newark isn't a focus for JetBlue. JetBlue is a minor carrier there but they do have Florida and Boston service out of there. But I agree maybe to some extent JetBlue wants all of NJ and Lehigh Valley to support their small operation at EWR. But I have a hard time fully agreeing with that and think they are so small there and EWR is quite far (over 70 miles) from SJ and outlying areas of Philly. Anyways, CLT and RDU service would be non-conflicting that way.
Delta is at many small airports in the Northeast, including many airports with fewer passenger enplanements/seats than ACY, with connections to DTW or ATL from those airports. They don't service ISP but it's understandable with a large LGA/JFK presence. However, ACY is quite away from that LGA/JFK presence.
ACY is low yield and South Jersey lacks industry. However, there are 2 million residents in the SJ counties (Camden, Burlington, Gloucester, Atlantic, Cape May, Cumberland and Ocean) and ACY is within 1 hour of a drive so it could be used more.