I agree with what was said above, however I think that there are several reasons, in addition to fleet commonality. It's all got to do with m-o-n-e-y.
The lease rates on the 717s were terrible. Yeah, AA could have renegotiated them with Boeing, however, that still leaves them with duplicates; they have the F100 and the 717 at the same time. That's a waste. The Fokkers are paid for, the 717s are not. That pretty much was the kiss of death for the AA 717.
There were only five MD-90-30s or so in the whole fleet, and it just doesn't make sense to maintain all of the infrastructure for just a fleet of five. Also, Reno had MD-87s which is another fleet difficulty. IT doesn't hold enough people, so you can't schedule it on normal AA MD-82 flights.
Reno did have the MD-82s also, but they were leased. AA kept them until the lease was over, and, not needing any more aircraft, got rid of them at that time.
The MD-82s have started going to Vanguard.
I do agree that AA cares about commonality a lot. But look at where they are. Including TWA, they were and still are flying a very diverse and consequently inefficient fleet. MD-11s and 777s. 767s powered by two different engines. 757s powered by two different engines. 727s. 737s. 717s. F100s. A300s. it's a mess.