I've been travelling a lot laterly. I fly with two airlines; one full-service and the other is 'budget' carrier. The flight is ~1.5 hours. I notice that :

both companies' plane are usually 70% to 80%
both companies' have about 10+ flights daily.
the full-service carrier offer free drink + peanuts
the budget carrier is about 30% cheaper
the budget carrier's ticket is slightly more flexible per fare class. (eg date/time/cancel/refund)

I don't see any other differences. Same custom service, leg room, seat width, check in / booking procedure, on-time departure rate, etc.

So my question is : why isn't the budget carrier 99% full, and the full-service one 50% full ?