How can anything possibly be worse then bum crap?
Like any good pestilence, the answer is multidimensional. But let's start with something we can all relate to: The old saying that "A little dog s**t goes a long way", meaning that when you step on a little, the smell is big for a long time and cleaning it up is a lot harder and more time consuming than you ever would expect.
Extending this concept, which is worse? A little dog s**t or a bad evil tempered and dangerous dog that s**ts all over the place and ruins everything it gets near, and which always seems to be under foot so you have to constantly avoid it and avoid stepping over it. And that dog stinks so bad that you want to puke when you are near it.
Extending that concept, which is worse, one such dog, or tens of thousands of them distributed so that you can't avoid them anywhere you go in business and commercial districts? And in concentrations so great that you will always have to avoid many of them no matter where you are? Think of zombie movies, but the zombies mostly lay around sleeping and being stoned, and are always accosting people verbally and often physically.
That's San Francisco. Just replace "dog" and "zombie" with "bum".