Originally, it was because we didn't think anyone would ever exile someone they could ban.
Then, after we thought about it, we decided to leave it that way because the point of exile is the power struggle.
Basically, if you can ban someone, you're a moron to exile them. But we don't try to prevent you from being a moron smart.
So in other words there is no reason. Perhaps the ruler wanted to disgrace the exiled for awhile then after being properly disgraced, have him banned.
