There's a reason for that, LGA has a fence around it, one of only two in the US (the other is DCA). Besides Denver, you can't fly more than 1500 miles out of LGA Sunday through Friday. This was put in place when IDL (now JFK) opened, to push the long distance travelers from Manhattan to the newer airport. When AA moved out of LGA, their new hub airport was chosen as DFW because it's 1466 miles from LGA, just inside of the fence.
What this means for you is that many very useful shorter flights are at LGA, because for the business travelers in Manhattan it's the closest airport. EWR and JFK get the medium and long haul flights primarily with more limited short haul service. And yes, DCA and IAD have the same relationship going on.