Tucson has always had two problems:

One, demand is highly seasonal.
Two, since the airport is south of the city but most of the city's wealth is concentrated on the north side, the airport suffers a significant bleed of higher-yield passengers to PHX.

I'm not sure what switching to US will get you. Except for a tiny number of flights to LAS, all of US' flights in TUS go to PHX.

If most of your flights take you east, American through DFW and ORD might work. Otherwise, there's always Southwest.

Sorry if PHX is not convenient for you, but United is the #3 carrier at PHX. They do offer a lot of flights there.