Technical progress moves in giant leaps during large wars. This rule applied to nearly all types of weapons and armament during WWII, including tanks. Every year of war raised the bar of requirements, and even the newest fighting machines quickly became outdated.
The heavy IS-1 tank was created as a response to the Tiger, but was obsolete by the time it entered production. The IS-2 that came to replace it had a much more powerful gun, but it too would need to be replaced soon. Today let us remember the tanks that were supposed to replace the IS-2 in 1944. Not a single one was ever built in metal, but work on these projects resulted in the creation of the IS-2 with straight upper front hull armour and the IS-3.