Although I work in EWR, my understanding of this is that JFK can only land simultaneous side by side IRF arrivals on the 31's, but because the 31's are not that far apart and that requires the 2 final monitor positions to be staffed at the Tracon, as well as extra staffing on the tower. They can run staggered 31's, as well as staggered 22's, but these require vfr wx so visual separation can be used, and thus are very complex operations due to the extra heavy chatter needed (controller has to first call traffic to each arrival, wait for the "we have it in sight" reply, then issue the "maintain visual separation, follow that traffic blah blah blah), while that is normal verbiage for 1 runway operation running visuals, when you double the traffic in a congested airport like this, it becomes almost too much to handle by one controller, and you can't split that into 2 finals due to the close proximity of the finals.

The difference between the NY airports and places like DFW, or ATL to name but 2 examples is that in those places the parallels are WAY farther apart, and they can legally run side by side operations without the need to have final monitors, or use visual separation, and usually involve different controllers working different finals with plenty of room to operate within their own airspace.