They are limiting A fares only because they think they can sell them for P or F fare cost. You don't mention P fares in your OP--UA uses these as a middle ground between A & F. You can frequently find fares that book into P on most routes that have a QUAUP fare--there are usually YUAUP fares that book into P.

I usually don't have any problem finding A availability when I need it. Maybe you are booking too close to departure? What are your usual routes?

The question is, if you can find A fares on US/CO, why not fly them? I know US's F service is not as good as UA's, but most people find CO to be comparable, if not better.