A lot of them, not all, have a supposed investment in the UFC and propping up the UFC, and Fedor was not only the target of a smear campaign by dana white and the ufc, was positioned as an ENEMY of the UFC, because they didn't get him. He's the one that got away, and instead of acknowledging that, he is shit on because thye didn't get to see him fight in the UFC.
tl;dr - They didn't get what they wanted.
Some people have a bad taste in their mouth of him constantly teasing that he'll go to the UFC then not doing it, some of which i think is the ufc's fault, some of which i think is fedor's fault, and that I understand. I have a bad taste in my mouth from GSP teasing that he'll fight Anderson, then never doing it, and it'd be unfair to not hold Fedor to that same standard. He opened himself up for a bit of criticism when he uses the UFC and teasing that to build up his next fight, which I maintain is to build up his students as he has no reason to keep fighting and probably doesn't get paid for more people tuning into a FIGHT PASS STREAM. Still though, it rubs people the wrong way. I think that the hardcore desire to see Fedor in the UFC is weird, why? Because he hasn't before. Okay, why want to see him fight in the UFC at 40? What do you get out of it? Just seems strange.