Just because a game's gone cheap, it doesn't mean it's not selling.
But lets take a lookie here:
NV is basically a port of Fallout 3 so your paying a new game price for a conversion of an old game.
It was released in a more bug ridden state than most Bethesda games and that takes some doing.
It uses an old engine that Bethesda never seemed to master, (even though the guys from Larian can make it hum along quite nicely).
Its a Steam activated title, might not put you off but it does put others off.
Forced patches from Steam breaks everybodies mods.
And now, so soon after release, its gone cheap, in fact, its half price.
Yup, must be sellin like hotcakes