Tiger has a bit of ROF advantage and is just more universal with Panther being more mobile, having vet0 range advantage and marginally higher pen.
The Panther can hold back the IS-2 decently, but it doesn't counter it as well as Allied TDs counter a Tiger because of lower penetration, lower range, lower ROF and higher cost. Once the IS-2 gets 50 range at vet 2, the Panther struggles quite hard.
When it comes to AT however, the Tiger is a much more reliable matchup to the IS-2, because it has similar penetration, but significantly higher DPM, has higher armor and health to tank return fire, and isn't too far behind on mobility.
At a max range engagement, The Tiger has a significantly lower TTK versus the IS-2 (~47,25s) than the Panther (~59,85s) while also being much more durable. Speed isn't very relevant here as both are faster than the IS-2 (and flanking to get rear armor hits is nearly impossible anyway). The only real advantage the Panther has is a small 10 range advantage, which is nullified by the Tiger's or IS-2's vet 2. The Tiger also has an easier time scaling alongside the IS-2 because of better veterancy.
Then, as Lago said, there is also the fact that the IS-2 can do both heavy AT and heavy AI duties, so it's way more cost effective to go for a Tiger that can do the same, as opposed to having to get a Panther and a Panzer IV (for 870mp and 325 fuel) to keep up with the IS-2's damage output. A faction like the Soviets has an easier time matching a Tiger I's damage output without a heavy of their own, as an SU-85 and a T-34 would only cost 220-280 fuel.
Reducing the IS-2's armor slightly would bump the TTK of Panthers and Jagdpanzer IVs and ATGs to something more acceptable, and a cost increase on the IS-2 would make the Panther a more cost effective choice. I think both would go a long way to make the Tiger I a less mandatory response to the IS-2.