Shadow Image Buff@Siohwenoeht
Reason is simple, due to the minor/major buff system the amount of debuffs that are applied to you is stunning, each skill can put even 3 negative effects on you. To be effective purge would need to remove at least 3 debuffs at a time otherwise you would have to spam it, and this I think would be a bit OP when combined with such great escape tool like shadow image, you don't want to make it so strong that it will be treated like rune cage of sorcs that has been nerfed into the ground.
On the other hand removing just movement debuffs on teleport skill:
1. Makes sense as it's about repositioning, nothing more.
2. Ensures that when used it always works the same (by removing the same debuffs not the random one)
3. Doesn't make it to strong. Like if you have on you 2 haunting curses and furry, you will still blow up if you won't defend your self.