Part of it is by comparison. The other Tier 9s currently in the game are the T95 (which is the only thing in the game whose front armor is truly inviolate against AP as far as I can tell), and the Object 704 (which has sexy sexy slope of 45 degrees or better and moves around a lot faster than other two), and the T30 (which has a full turret and a similar amount of armor on the top bits).
The other part is, as you guessed, it's mounted on a Tiger I chassis despite having a giant casemate on top, and the Tiger chassis is actually pretty high so it's hard to get a hull-down.