Because its custom changes and they would also affect PvE.
3.3.5b should only be a thing on a cross realm.
Upon entering the cross realm for a BG or Arena the game client would change from 3.3.5a to 3.3.5b.
In this case the main realm would still run on 3.3.5a and PvE wouldn't be affected.
But I'm afraid that developing this would be extremly challenging, if not impossible.

