They can easily schedule BYU in basketball and football, if attendance is what they are after.   Some have, some haven't. 

I wish Dave Rose would schedule more MWC schools in basketball---they are a huge upgrade in game excitement compared to the boring WCC.

but the MWC is using it's leverage to get back BYU football.   They think that if BYU is left in the crappy WCC (without Gonzaga) and they put a condition on BYU that it can only join for all sports (not just basketball), that it might force BYU's hand.  If they allow BYU to join for basketball, and give a scheduling agreement to BYU, that only helps BYU, and the MWC has lost all of its leverage.

Sadly, it looks like the WCC will lose Gonzaga, and BYU will stay in the WCC.