Around that time there was a huge debate on whether or not tanks would even be relevant on the battlefield in the future, due to the increasing power of man-portable anti tank weapons. Soviet tank losses in places like Berlin (often to civilians firing Panzerfausts from rubble) were surprisingly high. Tanks that used steel armor simply couldn't keep up with HEAT rounds being developed at the time, regardless of thickness.

Also modern battlefield doctrines (and post war budget realities) favored using multi-role tanks (MBTs) rather than specialized tanks such as assault guns/tank destroyers/heavy/medium/light tanks as was done in World War 2. Germany realized that while specialized vehicles were particularly good at what they were designed to do (think of the Stug III) they performed pitifully when pressed into roles they weren't designed for, such as taking the place of a medium tank.

The United States also had issues, such as their doctrine calling for using medium tanks to support infantry, while tank destroyers would be used in the anti-armor role. However they quickly discovered that it was impractical as the tank destroyers often weren't where they were needed. All of this basically sums up to the fact that it is better to have a "jack of all trades, master of none" tank that can be decent in any role it finds itself in, rather than having a vehicle extremely good at one thing but terrible at others. For this reason, heavy tanks like the IS7 simply fell out of favor.

To make a long story short it was a lot less practical at that time to use heavy breakthrough tanks, and the idea of the well-rounded Main Battle Tank was really taking hold. The Soviets just decided to extend the lives of their previously existing heavy tanks rather than introduce a whole new model.

TL;DR The big heavy expensive IS7 couldn't do a whole lot more than a small cheap T-55 could.