Simply put
When you joined you knew you were not getting housing allowance and that you would never get housing allowance - yet you still joined CX = so you do not have a leg to stand on.
As a similar example - those that joined without receiving a "travel fund" still joined CX. I rarely hear any complaints about the lack of travel fund and I have NEVER heard the AOA try to get a travel fund for those who do not receive this remuneration.
The AOA should be fighting for a general "salary increase", and not for an allowance for a small segment who knew what they were getting into when they joined.