Series change with sequels; that's an unavoidable fact of life.
I can respect that you feel it doesn't feel like old Fallout because it's not turn based, but if you're objecting on story or design grounds that is a purely subjective area.
Unless you mean you feel only games set on the West Coast in that area should be numbered?
It goes beyond not being turn based. F:NV feels a lot more like a true sequel to F2, because it makes a lot of references to places and characters in F1 and F2, and actually talks about the Chosen One (player character in F2) and his/her actions. It also brought back game elements from F1 and F2, like a reputation system and different kinds of ammo, things that were removed from F3 for some mysterious reason.
Yeah, in a way Fallouts not set in the West Coast really shouldn't be numbered. Why should they? The numbered games are a series - Fallout 1, then Fallout 2 which is a direct sequel, and whoops, a game called Fallout 3 which has nothing to do with those two games. It's like Far Cry 2, what does it have to do with Far Cry?
My point is, F3 has nothing to do with F1 and F2. Doesn't make a bad game by any means. It has nothing to do with the gameplay being different. The end.