Because a lot of bad people circulate among these groups. It's why being homeless or living in poor communities is so dangerous. Being homeless or poor doesn't

Because a lot of bad people circulate among these groups. It's why being homeless or living in poor communities is so dangerous. Being homeless or poor doesn't make you a criminal, but it does attract criminals and criminal behavior--which is largely opportunistic. If a block is full of drug users and people loitering, the thief smashing windows won't stand out, or if he does he can quickly blend back into the streetscape. Proximity also makes it more likely that those predisposed to thievery will band together, creating a cycle of escalating criminal behavior--a virtuous cycle, so to speak, not unlike how Silicon Valley works because of the density of engineering talent. IMO, one reason why the drug epidemic has exploded in SF is because of the creation and centralization of so much homeless housing around the Civic Center and SOMA. Nothing is more certain to exacerbate addiction and drug use than to put drug users in close proximity. And anti-social behavior invites more anti-social behavior, particularly crime. The Tenderloin always had it's share of problems, but the problem was primarily poverty, not extreme social dysfunction. Violence and property crime were relatively rare as compared to today. Drug use had been tolerated for decades, though the unwritten rule was that people had to make some effort to hide it, like shooting up in the alley. The erasure of this one very simple quid pro quo, the removal of any serious threat of consequences (i.e. the prospect of being forced into rehab if you invited arrest), and the dramatic increase and centralization of the addict population thanks to Mayor Newsom's and Mayor Lee's creation of over 10,000 units of permanent housing for the homeless, has created chaos.