Well, I think it's because it had too many QTE sequences and although it's a decent co-op game, it didn't feel like a proper RE game. If it was just an action game in general, I think it would have gotten better reviews. When Capcom tried to deviate too far away from what the series started out as, this is when the purists started to think they were selling out. Essentially, if anybody wanted a Call of Duty type game, they would play Call of Duty.
Like, if you watch these two videos to compare the older and newer styles in the franchise, you'll see that the first game has a massive abundance of atmosphere, and RE6 is just a shoot and loot game with added co-op, and it also feels more cheesy than creepy.