Just some theories they have thrown out there the last few years:
Lack of revenue compared to the rest of the P5 makes it harder to compete for top coaches.
In terms of exposure they have by far the worst TV deal of the P5 conferences, and it's not even close. People living in Pac 12 states can't even get games on TV. This hurts in recruiting.
USC (the top football school) getting nailed for sancions from which it has never recovered, and now Arizona (the top basketball school in recent years) getting caught up in the Addidas scandal.
The Pacific time zone means too many games are played when everyone else is asleep.
Scheduling that has given the conferences top teams short weeks to prepare for big games which every year has led to a major upset. Washington and Stanford has been victims of this the last two years.