Entirely because of allied TDs. From p4 to KT all they CAN make is TDs and since it's the same unit you can't catch them off guard.
IS mobility also plays a role, but yes, axis core armies consisting of stock super heavy tanks doesn't help balance, because it means that all allied factions need to have a reliable tool against these stock super heavies.
It could all be balanced if Relic was capable of letting go of that "bigger is better" mentality for axis armies as there was a plethora of lighter armor that was equally effective, which led to current situation where allies have only 1 answer for all armor and no, putting in a unit exclusively to deal with heavies and heavies alone is a horrible idea, snek, as it creates bloat that serves no real purpose.