It's more a product of the ridiculously high PPV prices than anything else. People aren't going to shell out the big bucks for multiple UFC events a year, especially when few PPVs are padded with big names. It doesn't mean the people aren't watching. They're just finding alternatives... like chipping in as a group, hitting the pub, or illegally streaming.
You can have 15 friends chip in on an event. Still counts as only one PPV buy. But again, this is what happens when you're charging 65 dollars a pop, sometimes twice a month.