With all the comments on where the next Envoy base will be, I started looking at where some of the other carriers fly from for AA. PSA flys from DAY, TYS, CLT, CVG, DCA. Piedmont from ROA, SBY, PHL. Just curious why the non AA hub locations for bases?

If one eliminated the "obvious" choices of MIA and LAX; CLT, DCA and PHL already have a AA WO presence, that leaves BOS, STL and PHX. Or is there a reason to put an Envoy base as a non AA hub like PSA and Piedmont currently do?