This is because of it's epic offensive move-set possibilities:
On the other hand, its stalling / hit-taking capabilities aren't all that strong. It just doesn't have the kind of moves it takes to be a wall. A tank, yes, but not a wall, except a physical wall, thanks to Iron Defense.
Besides, it has a typing, that sadly, gives it a 2x weakness to most of moves that commonly used whose types include, but aren't limited to Fire, Ground, Ghost, and Dark.
I must add here, however that it does have a lot of resistances, but sadly, in the meta-game the afore-mentioned moves are the most common, with the exception of Fairy moves.