I think another big reason ratings are falling for the UFC is because of oversaturation. I'm as big a fan as there is, but there are WAY too many events now. The cards are watered down because of it. The 4th and 5th fights on the main card ten years ago are headlining the smaller events now.
Add to that the fact that a large portion of UFC fans, the mouthbreathers specifically, don't like the smaller weight classes because they generally tend to lack the knockout power larger fighters possess. Their speed and skill is taken for granted. For example, Demetrius "Mighty Mouse" Johnson is probably the best pound for pound fighter in the world right now, and he can barely draw fans to the free events on Fox.