It's not just ORD, compare the number of flights and size of aircraft that fly between JFK and BOS, DCA, ATL,PIT, DTW, IAH, DFW, CLT, etc and you will see little or no service from JFK to these cities.
JFK is the hardest to get to from Manhattan, and it takes twice as long to get to JFK from Mid-Town as it does to get to EWR from Mid-Town even though they're the same distance.
JFK is for long haul and International flights.
Of the three NY airports (EWR,JFK,LGA) JFK has the smallest percentage of business travelers, business travelers are the folks who pay the big $$$ for air travel.
Eastern Air lines flt # 701, EWR-MCO Boeing 757