This is literally the crux of the issue, and the single worst way you could try to balance any tank that relies on it’s armor. Such vehicles are inherently very sensitive to the tier they’re played at, for obvious reasons. When combined with other factors like a weak gun or appalling mobility, it often results in a very binary experience when playing it. They effectively cannot be balanced in a way where they’re usable in uptiers without also being ridiculously dominant in downtiers.
So, for the sake of game health (And because, again, there is no-one standing behind you with a gun to your head, forcing you to spawn it in uptiers), the best way to balance such tanks is to balance them around downtiers.
This ensures the best chance that the tank is balanced for the vast majority of players.
This is a blatant “whataboutism”, but whatever, I’ll bite.
The E-100 is a tournament prize, and thus only given out to the very best players in the game. You basically cannot acquire it any other way.
The Maus merely requires reaching mid tier Germany and waiting. Any player can get it, even if they’re relatively new at the game.
Comparing the two head to head, the E-100 is basically just a Maus with better hull armor and marginally better mobility. It’s undeniably a straight upgrade, but enough for a full BR step? Obviously, having not played it, I can’t say for sure, but I’d wager not. It’s still hampered by being a slow, armor reliant heavy tank right at the very end of the armor meta. Were there decompression, sure, but as it stands now, there’s really no reason.
The KV-220, conversely, is a vehicle that was available in one of the earlier grinding events, and thus a lot of people got. It’s also available as a coupon from the SL crates, meaning there’s always a trickle of them available at all times.
Sure, it’s comparably harder to get a hold of than tech tree or premium tanks, which would inflate the statistics. But this only puts it into Class 3P/T-10A levels, not 279/IS-7 levels (Which are specifically much rarer drops from the SL crates).

