[samrohan]

If it was not to make the story more interesting, I am sure Tolkien would have never allowed the Old Wizard to fall down after being hit by the Balrog's wip. He had already defeated the evil being and should have been able to invoke a spell not to fall down.
My guess is that Tolkien wanted to make him more human, though being so powerful he just lost his balance.
If he could defeat or stand up to Ring Wraiths he could certainly stand up to Balrogs.