Basically everyone hates him, he's been pushed too hard and too early, if WWE took the time over a couple of years to build him up, he could be a very good champion.
The fact that WWE are actively removing anti Reigns signs from the crowds, muting boos and piping in fake cheers really shows that the fans clearly don't want this.
Plus his booking makes everyone look like shit, AJ beat him with a chair, delivered his finisher twice, once on to a chair and had interference from the club and still Roman kicked out and finished the match with just a single spear.
Neither guy comes out of it looking good and hey presto you've got a mutinous fanbase on your hands.