A good point, why is the is2 rarely used then? Because no guards in any of the docs with is2?
That is hard to say, it seems that people like to play it safe and play what ever is meta so a unit not used does not mean that the is UP. It might even be OP.
Dhsk was OP for years yet people would go for the maxim meta, osttruppen where not used (after their original nerf) until helpinghans brought them into meta and so on.
Currently Guards are OP (at least in my opinion) and they can be found in great commanders with other powerful abilities like PPsh and KV-1 so it might have something to do with not seeing IS-2.
On the other hand Soviets can win without putting all their money on single "game changing" unit that will cost them their game if they lose so why bother taking the risk?
At least that is my opinion.