I don’t know why all the others go to EWR, but the few times I’ve needed to fly commercially recently (pre-covid, anyway; I haven’t flown commercially since then), I’ve used EWR specifically because it’s the easiest to get to/from with the Amtrak Newark Airport station. Stewart and Atlantic City are both farther and less convenient for me to get to/from. So it’s not as simple as just having more flights use those other airports.