I think that's because of the matchup with the Panther.
The Panther beats the Comet (800 HP), can take on the Pershing (960 HP), and generally loses to the IS-2 (1040 HP).
This makes the IS-2 the most impactful heavy tank on the meta despite not being that much stronger than the Pershing on paper. If the opponent wants to counter it without going heavy tanks themselves they need a Panther and something else. They need to expend more resources on AT units than the generalist IS-2 costs.
Viewed that way, it's easy to see why the IS-2 is such a popular pick in 1v1: it doesn't have a nondoctrinal counter.
Then we have the Tiger. The Tiger has 1040 HP, making it better in a heavy tank slugging match than the Panther. This makes it a better pick than the Panther even against the Pershing: anything the Panther can do, the Tiger can do better.
In summary, we have these two problems:
- The Panther cannot deal with the IS-2.
- The Tiger is a better anti-heavy unit than the Panther.
What if we changed that?
Reduce the Tiger's HP to 960. Adjust cost or buff as necessary.
Reduce the IS-2's HP to 960, or rework it into a more expensive tank in the vein of the King Tiger.
What if we cut the IS-2 and Tiger to 960 HP, same as the Panther and Pershing? The Panther can then take on the IS-2 in a one-on-one battle, and the Tiger stops being a better pick than it.
Now that the IS-2 can be countered nondoctrinally, there's less incentive to go IS-2 every game.
Now that the Tiger isn't a better heavy counter than the Panther, there's less incentive to go Tiger every game.