They want to set your expectations if you're connecting onto one of those 777s in Y (seat width on those are terrible)

Regarding another poster's comment, I really doubt most flyers consider how far out of the way a connecting airport is, or not at least with PHL vs JFK. It's not like JFK was losing connecting pax to PHL. Or at least not that I'm aware of. It was a purposeful move by leadership to keep JFK more Premium O&D oriented. Something flyers and AA do consider is price. Specifically for AA, it's not just about profitability, its about where they can make the *most* money flying a certain aircraft. It appears like they have found that JFK is too competitive and too expensive and PHL is just better for connecting TATL pax.

Edit: If ubernostrum was referring to all the ATC/weather delays that come from geography, then yes, JFK is terrible.