They expanded and there are too many events and not enough stars.
To build stars, they need to build them off of other stars. You put a card with a bunch of semi unknowns fighting each other, even if they are skilled, a lot of people won't be tuning in because they don't know who these guys are, especially at 50+ bucks.
People watch MMA for different reasons, those that watch for familiar faces have less reason to watch these days as there are a crap load of fighters and events and the familiar faces are spread too thin among too many events. This in turn gets them less in tune with the next generation of fighters and they become more and more out of touch like a guy that misses a few episodes of a tv series and then just starts blowing it off completely.
Anyways, even if fans are getting bored, maybe it is still popular/profitable enough to keep up the current pace. If that is the case, I don't think the UFC gives a shit if we are getting jaded, and will only pay attention if that lull becomes too great.