I think a plot of people equate writing with story and plot, but New Vegas doesn't really have a lot set main story or plot and is left more up to the player to determine. The strength of the writing in the game is the characters, factions, world, quests and small side stories that set up the framework for you create your own main plot and story, but most players new to series with Fallout 3 aren't at all used to something like that and just assume the writing is weak because they expect it force you into some sort of "epic" story it is trying to tell like in most other games.
Also a lot of people wouldn't know decent writing if it bit them in butt. Look how much crud like Twilight sells, Fallout 3 fits right into that style of hack writing.