The Soviet Storm documentary boasted of the IS-2 tank that it had been so good it remained in service until 1995. All I could find online was that the South Ossetian Army operated some IS-2s until 1995. Why were these antiquated tanks still in service?
For the same reason insane amounts of Mosin and Mauser rifles were mothballed, to arm the third echelon of WWIII. The D-25T was a powerful cannon that could fire a round that was effective against second tier Western armour, plus the more powerful M-62 gun could be dropped into the D-25's gun mount. In a worst case scenario, these tanks were better than nothing.
The second reason is bureaucratic inertia. This is, of course, anecdotal evidence, but I heard a story about Tsarist cannonballs that had to be retained because the quartermaster wrote "keep for 100 years" in the records. The process to write them off ahead of time was complicated and lengthy.
By 1995, Russia needed money much note than weapons for a war that would never come. Cheap Mosin and RC Mauser rifles flooded the market and the harder to sell stuff like tanks was scrapped or sold off at scrap value.