Don't know why ISP can't attract or keep the other carriers. They've had & lost hub service over the years to ORD, CLE, CVG, EWR, IAD, PIT, CLT, ATL plus a number of other northeast destinations (BUF, BOS, ALB, SYR.)
I think marketing has a bit to do with it. The name MacArthur doesn't resonate nationally, but if they renamed it Suffolk County Airport, that could help. There's a ton of population from mid-Nassau & east that they should be able to skim traffic from.