Free sandwich?

Seriously, though, it's going to be price or schedule.

Out of SFO, for example, CO has 6-8 SFO-IAH flights, while UA has one (1). If you need to go to IAH, CO will likely have the best schedule.

UA also has a big hole in the Caribbean, Latin America, and South Florida (though you could argue that Latin America includes South Florida, but that's a different thread. )

Plus with CO's IAH hub, even for destinations served by both carriers, some southerly destinations are more efficiently served via IAH than IAD or ORD. If you want to get there as quickly as posisble, CO via IAH may be the way to go for West Coasters.