9x18mm is the probably the most powerful round that can be safely deployed in simple blow back pistol with reasonable size and weight, and felt recoil.
Simple blow back pistol is easier to manufacture and maintain than other short recoil pistols. (That was why I think they made the switch.)
Also easier to train for mass conscrip army.
Also easier to fire in short full-auto burst from Stechkin. (Can you imagine shooting full auto 7.62 in comparison?)
By the way, Leroy Thompson raved about Stechkin.