I don't mean this in a snarky way at all, because this is odd, and I would also find it very annoying- but have you contacted AA with these same data and asked them why?
They can claim "weather" all they want, but if other similar airlines (UA are certainly not taking any more "risks" than AA) can get it done, they should be able to have some answer as to why they're not. It's going to take more than one email, and I'd even suggest that you request a call from the executive offices about this- it's not something you'll get a simple answer to in email. I've done this with similar, complex issues (rarely, of course) and it has yielded good results.
Cheers.
I did explain the disparity to the EXP desk when having them switch me over to a UA direct flight tomorrow morning. Since UA did not do a cancel-o-rama today between LGA and ORD, there is plenty of availability on UA. AA is zeroed out all day tomorrow.
The EXP desk claimed that all airlines are instructed to cancel a certain percentage of flights by ATC. According to the EXP desk, UA chose to cancel flights on different routes. Not sure I buy that explanation as LGA-ORD is a very high load factor route for AA. I can think of many other ex-LGA flights that could be cancelled with far less passenger disruption (hello AE).
A letter to AA's executive offices is probably in order.