Cause purges put fear in party functionaries, and guy appears (who signed orders as well, mind you) saying no more purges, you can do whatever ;) and thus cements his power. Who cares it shoots any credibility dead in the world (and there is a difference between owning mistakes and exaggerating them to cast shadows over predecessor).

Cynical theater fortifying power inside the party (cause no reprisal will come for your indiscretions), and crashing it elsewhere. But the blame lies on stalin as well, especially after ww2 the guy could have hit some system stability literature, he was close to unimpeachable.