Because major airports are typically ALOT cheaper to fly in and out out of and there are more flight schedules.
I find that United uses United Express connections into Bend from Portland, Seattle, SFO as leverage to get you on a United flight. That is, they tack it on cheap to their flights, but not other airlines. I seldom find flying out of Bend to me much more expensive than a big airport.
Of course, then you end up on United......