Her hands were tied. She had to wait until things are bad enough that the board of supervisors - the majority of whom are hostile to her - get forced to play along.
People saying she has been forced by tech exodus or whatever are betraying a lack of understanding of how sf politics works. The recent BOS decision to reject a couple of housing developments on fairly spurious grounds has really shown how stupidly political and out of touch the BOS is; to make them get along with anything requires a crisis.
Side note: my mental model of the BOS is the Pawnee town council from the TV show Parks and Recreation; the real life folks seem to operate along depressingly similar lines.
