Maybe its because of his weird British accent? He hasnt said a bad word against Miller in any of this pre fight hype, yet people still find a reason to hate the guy. Where the hell did he call out Anderson Silva?
I've wrote essays about why Bisping is disliked before, but here's another...
He come off like a bit of a prat again at the press conference, and even realised and said so himself. He has such a high opinion of his career and his wins, stating he has beat 'loads' of black belts and smashed their faces in. He doesn't know how to talk himself up without rubbing people up the wrong way. I don't really dislike the guy I just don't think he's one bit likable. Not funny, not sincere and often claiming he's much better than he is and done more than he has.
When he actually beats some good fighters it might not be so stupid when he says stuff along the lines of 'ive beat loads of black belts before'. What black belts is he talking about? Denis Kang? Great, he's no way near the top of the MW division and is Bisping's only notable win. When you hear fighters saying they're gona knock their opponent out, they usually can and have some good wins on their record. Bisping is mid tier, can't beat a top MW and still somehow decides he's going to talk like he's Chuck Liddell in his prime.
There comes a time when the talkers in the sport have to back their mouth up, and Bisping never has and imo, never will. That makes him annoyingly arrogant, because he just talks, tries to look like mr. mean and never delivers when it matters.
Add to that the disrespectful stunts he's pulled before, you have yourself one disliked fighter. And there's always an excuse; heat of the moment, everyone makes mistakes etc but he's a grown man who at too many times has acted like a prat and annoyed people. He tries to keep it toned down these days but it's a bit too obvious, and signs of his true character still shine through on occasion.