Because players can now cast spells and other abilities while in Passive. If they didn't implement this change then mages would just stick in passive gaining MP just the same as they do in passive now only regaining MP at the same time as healing/nuking.
I figured they already implemented a counter-measure to this by making it so you can't regen MP if you perform an action or while you have hate.

So imo this can't be the reason.