There's a long history behind that, amplified in the 50s by Chronicle columnist Herb Caen, who named a book of his collected writings "Don't Call It Frisco". But he changed.
Here's a pull quote from the article linked to below, from Caen's alma mater:
"But t while still in the first half of his career, Caen started walking his position back. In a 1965 column he wrote: “Adolescence is believing that ‘Frisco’ is a racy nickname for a city; senility is automatically saying ‘Don’t call it Frisco’; maturity is figuring it doesn’t matter all that much.”
During Caen’s final columns in the 1990s, when readers were still sending Frisco items, he fully defended use of the word. One of Caen’s last Frisco mentions before his 1997 death was written Sept. 6, 1995: “The toughest guys on the old S.F. waterfront, neither rubes nor tourists, used to call it Frisco, and no effete journalist would have tried to correct them, either.”
https://www.sfchronicle.com/bayarea/article/frisco-san-francisco-17411378.php#:~:text=One%20of%20Caen's%20last%20Frisco,to%20correct%20them%2C%20either.%E2%80%9D