MMA fan for almost two decades here.
It's not all the UFC's fault. Let me explain.
The UFC is indeed doing a horrible job of harvesting new stars and marketing them properly. On top of that, they are putting on far too many events that even their bloated talent pool cannot fill. There are several cards a year that come and go with barely a single star on them.
The problem this creates is hardcore MMA fans used to want to see an entire card. They knew every fighter's name from the undercard to the main event and almost all the matchups were intriguing, at least on paper. Today, even I don't know half the guys and as a result, don't care.
Their global strategy is working, though. MMA is growing in many markets to more than make up for any losses at home. So while they don't have the same draw in the US, their Fight Pass and TV deals abroad are killing it.
This is good for them but bad for the hardcore fans in general.
And when it comes to TUF - the show is stale and terrible and they have not found a great solution. What is troubling is that when they do find something that produces a quality show, they seem to abandon that idea in favor of another gimmick or trying the same thing over and over, just with a different weight class.
Now for what isn't their fault - fighters just don't fight like they used to in general. This can be blamed on many different factors, but I think it ultimately comes down to three things:
1.) Money. The fighters are making more than ever and the guys at the top are especially in a position to fight "not to lose".
2.) Skill gaps. Though you have extremely dominant champions like Jon Jones and Rousey, everyone under them has a smaller skill gap than ever. Due to this, fights are much more competitive and you see less finishes overall.
3.) MMA has evolved. Because the 10 point system isn't going anywhere, fighters have learned to game it. When you combined the smaller skill gap with the need to win (lose a few fights in the UFC and your career is over or Bellator'd), fighters now more than ever are playing it safe.
There are many more reasons why my interest has lowered over the years, but it's mainly due to not being excited about cards anymore. People are shitting themselves over how good UFC 187 looks, for example, but this kind of card happened four times a year minimum in the past. Now we're lucky to get one mega-card a year.