http://www.modernluxury.com/san-francisco/story/arise-tender...
Quoting:
[...]
But the event that truly created the Tenderloin of today took place in 1980, when developers proposed three luxury high-rise hotels in the neighborhood. Residents and nonprofit advocacy groups organized to keep the big hotels out. The high rises were built, but in the aftermath of the fight, the city passed two crucial laws. First, the city made it illegal for owners to convert SROs into tourist hotels unless they also replaced the low-income units or paid into a fund for affordable housing. Second, it passed a zoning law lowering the height maximum in the Tenderloin from 30 stories to between 8 and 13.
These laws made it virtually impossible for developers to “Manhattanize” the Tenderloin. Meanwhile, the nonprofits, which had lobbied for the laws and led the fight to keep out the big hotels, themselves began to buy or lease buildings. These nonprofits, which include the Tenderloin Neighborhood Development Corporation (TNDC), the Tenderloin Housing Clinic (THC), and Mercy Housing, now own or lease more than 5,000 housing units in the Tenderloin. The city plays a critical role too: It has loaned the nonprofits money to acquire or rehab about 50 properties with 4,000 affordable units, and it holds or finances master leases on dozens of other such buildings. “The Tenderloin is the only place where the nonprofits own the housing,” says Field. “This protects the poor people in the neighborhood from changes in demographics.” It also gives the nonprofits a vested interest in keeping their clients from being dispersed.
[...]
Combine that with rent control, resistance to new, higher density development and an influx of high paying tech jobs, and San Francisco's current situation makes a lot of sense...*
* To be fair, policies from the '80s that closed asylums and dumped mentally ill people onto the street contributed, but SF certainly seems to have worked to maximize the effect.