Because I don't think it would be ok for communists to rename it to Tsaritsin. It would be against their "religion".
I'm sorry if I sound rude, but no-one really gives a damn if you prefer to call it X or Y. The name stays the same as it is officially known St.Petersburg.
Besides Peter the Great probably stopped turning in his grave after 1991.
But, again, OffTopic.
Классная рама. Удачи с продажей и с наступающим Днем Победы!!!