I don't know that there is less interest. They have many more options to see fights now and many more events. They don't just have the UFC once a month. Now they have the TV show, Fight Nights on Fox/Sportsnet, etc.
Also, a lot of fans are no longer paying for PPV and simply streaming it over the net for free.... I personally haven't paid for a PPV event in at least 2 years.... but I'm still watching. In fact, I don't know of a single friend of mine that pays for it any more when before several of us would each event.
I will agree that the UFC has a lack of really big name or interesting talent. They are losing many of their stars all within a few years. No more Chuck, Tito, Lesnar, Silva, GSP, Penn, Cro Cop, Hughes. The talent is certainly there, but the characters aren't.