It was a very active choice by them to try and fit a race that was an entire army into a side note of the Alliance. They managed to do the same with the undead but that at least was explained by fragmenting the Forsaken from the Scourge.
Even Ashenvale being contested is kind of murky in lore. Grom is losing his fight in Ashvenvale and needs to drink Mannoroth's blood to win. He later kills Mannoroth and fel orcs disappear. Tyrande and Malfurion return to Ashenvale between the events of the battle for Hyjal and TFT. Clearly it was well within their ability and power to remove any orc from Ashenvale at that point.