I think that started precipitating with the Ganados in general, really. The only thing that RE4 had in common with RE2 (or the other previous games, for that matter) was Leon and Ada. And that was about it
What utterly irks me is when my Nintendo fanchildren friends like talking about Resident Evil 4 is the best ever and they haven't a clue about any backstory and haven't even played the previous ones. They know it was about zombies, and that's it.
I mean, there weren't even flashbacks or anything in RE4, it might as well have been a totally different game from another franchise and that would have worked.
Not to say it was a bad game, it just allowed the opportunity for further games to be totally disconnected from it and even though they tried hard, RE5 was a step in the same direction.
EDIT: Come to think of it, the Trevor family might have been the most tragic thing in the entire series. Wesker's Report 2 brought a whole load of "hoshit that really sucks" into the backstory. Research guy goes for a bit of a travel with his family, meets up with an asshole who kidnaps his family and does horrible shit to all of them just so he can get something out of the Progenitor. =/
EDIT x2: I'm not sure if it was Wesker's Report 2 or not. Goddamnit, I read it like years ago and there was an official Capcom page for it and everything and now I can't find it on the net anymore ;-;