The turning point for San Francisco was in the year 1995 when Terence Hallinan won the election for District Attorney. Hallinan was the first defense attorney to become the lead prosecutor in San Francisco. He immediately stopped prosecuting crimes, scoring the lowest winning percentage for a California DA by the time of his reelection in 1999.
Hallinan was defeated in the 2003 election by Kamala Harris, and the city has been on an irreversible track ever since.
San Francisco seems to have turned away from having prosecutors who prosecute towards defenders of criminals in the DA role.
-PJ
Being a district attorney has always been seen as a steppingstone to higher political office. The goal usually was to keep your nose clean, investigate your future opponents and only prosecute safe, easy cases. And when you are in a heavily Democratic state, you must also show your fidelity to liberal principles like excusing crime, looking the other way at corruption of your fellow dems and sounding right.