This is a good article about some of the UFC viewership trouble lately. Consider that in the last year you've seen Ronda Rousey, Jon Jones, Anderson Silva, and Connor McGregor either leave the sport or not compete for one reason or another then it makes since that the large PPV buys haven't been there.
Additionally, and the article doesn't mention this, the UFC has had a lot of younger like Paige Vanzant, Michelle Waterson, Sage Northcutt, and Cody Garbrandt not pan out as big time stars due to losing performances. I am not in anyway saying the company is in trouble but rather its coming back to reality after the Rousey and McGregor phenomenon have calmed down.