I think it's called nostalgia. I have played all the Fallout games, well OK I haven't finished tactics and probably never will and haven't got to NV DLC either but other then that I know the games pretty well.
So let's get to the point, yes 3 does not really fit into the fallout universe but I rather like it it's mostly dark and grim like post nuclear holocaust world should be 2 was a little more grim then 1 but NV went back to the goofy fun territory.
Story, 1 and 2 had better stories then 3, maybe in strict story book way yes, but that's the thing it was like reading a book, it wasn't a living world NV has it story stained by the goofy fun other than that I have to admit it's somewhat superior to 3.
Gameplay 1 and 2 have one big problem for me, turn based combat, it sucks so much ass, but then again I understand that there weren't really any viable alternatives for the turn based combat, but I still can't understand how is that I have 80 something % chance to hit a target and all six shots miss and it happens three times in a row, 3 is a perfect middle ground NV has those annoying collecting and crafting thing that you only really need in hardcore mod if even there, weapon mods a are a nice touch.
Game world, 1 and 2 are small technical limitations I understand, but still small what I don't understand is that how is NV smaller than 3, I don't actually know that it is smaller but it feels a hell of a lot smaller, also it took me abot 25-30 hours less to do everything in the main game in NV.
Overall, 3 good not perfect but close NV some steps in right direction some in not so right 1 and 2 may have been good games at the time but are old and ugly and date now, just like 3 and NV will be in 10 years time.