The problem arises when aircraft are told to start stacking due to various reasons,an aircraft may have to remain at a particular height for a considerable amount of time,if therefore you restrict the window of altitude that aircraft can fly in it can become very congested at particular heights.Also you cannot depend that an aircraft that is in a holding/stacking position will not be effected by ash if it is at the same altitude for long periods,the ash is falling at different rates in different areas.
Hopefully you get what i mean.