Other issue with PHX-EGE/ASE is that no one from SFO/LAX is going to connect over PHX (or SLC for that matter) to get there when there are ample nonstops. And those are the vast bulk of the west coast traffic heading to those destinations. The demand is just hugely concentrated.

Service from PHX is both thin as an origin, and to the extent you can pick up connections, they are even thinner. There might be a few markets - SAN-ASE or SNA-EGE or something that have a bit of demand - but everything else is just scraps, and a lot of those people (especially EGE traffic) is just going to fly to DEN instead. Its tough to make it work especially when UA has the clear schedule advantage with DEN connections - especially helpful with the notoriously WX-heavy ASE. Don't forget the high operational expenses/issues when stuff hits the fan during weather events.

Same reasons DL doesn't serve EGE/ASE/GUC/MTJ/HDN from SLC anymore, when those routes seem obvious. Minimal ex-SLC demand and the connecting demand from the west coast isn't worth fighting for. And from the eastern part of the country, its a long enough detour that theres a real yield loss vs DEN (or DFW frankly).