Presumably NYC-CHI is a competitive market, given that TZ, DL, UA, B6, AA, and CO all fly it. So perhaps the margins aren't there. All they really lose otherwise is a few express connections at LGA not available from other hubs. They can pick rhose up on UA codeshares anyway. But not from their website ...

Anyway, at 730 miles, 3 hours seems a bit pessimistic. Although takeoff queues at LGA can be pretty bad.