The dislike for Cormier has always been strange to me. I don't think it's one major reason, more a bunch of little reasons.
First, I'll just get it out of the way - he's black. It's always been harder to become a star in MMA when you aren't white.
Second, his body. Doesn't look like the high-level guys usually look.
Third, in his two biggest fights he got run over by Jon Jones. Lots of people have only seen Cormier fight a handful of times and two of them he got beat up by Jon Jones.
Fourth, as has been already mentioned doesn't have the typical MMA personality. He's not a cocky, trash-talking type.
Lastly, he's been matched up versus many popular fighters during his time in the UFC. Dan Henderson, Nelson, Anderson Silva, Anthony Johnson, Gus, Jon Jones. All of these guys are name guys that the fans are going to be cheering for regardless of who they were fighting. Even when he fought Patrick Cummins, people were cheering for Cummins because he had a cool back-story and took the fight on short notice.