5 is actually a good action game, but don't feel like RE at all, and 6 had some good ideas but was mostly boring.
The thing is, until RE4, it was a survival horror franchise, now it's action.
1-3 was all dark and mysterious, with real zombies.
4 was still dark, still mysterious and with a great adventure feeling (some people started disliking it since 4, but it's the best one), and with some rpg elements (increasable life, weapon improvements, treasure "hunting", shops).
5 and 6 were not even close to how good 4 was, so that plus the lack of survival horror feeling like the old ones make people don't like it that much.
Again, 5 is a good action game, but having RE as it name make it seems bad.