Updated Oct 8, 2019
It’s partly about two physical layout issues – one general, and one specific. Plus a perceptual issue.
San Francisco is very compact and crowded. Lots of very different people, crammed into a relatively small space. Many of whom take public transport, and also walk around. This includes tourists and other visitors.
Walk down Market Street during the daytime. You will see a large number of people, of all kinds. You won’t necessarily really register and remember the large number of yuppies, hipsters, tourists, etc that you pass by. But, disheveled-looking bums hanging around, aggressive beggars, visible drug use, urination, etc, will make a much stronger impression. Even if that is a relatively small per-capita percentage of the city’s population.
Compare that to a view of Los Angeles, where everything and everyone is spread out. And that view may also be through the windows of a car (including if you are a tourist or other visitor).
Next is the specific location of homeless services, such as soup kitchens, and many of the SRO buildings that San Francisco uses to warehouse the homeless and borderline-homeless. And, of course, some of the street corners where you can openly buy crack and heroin. You can also do various activities such as sleeping or defecating on the sidewalk, without being in front of the homes of any yuppies who might complain to police. This concentration is a charming neighbourhood called the Tenderloin district.
The Tenderloin is conveniently located a very short walk from Market Street. Including the Market and Powell cable car turnaround, which attracts large numbers of tourists. It’s also next to the Financial District, as well as the areas where a visitor might pass through on the way to Chinatown and to North Beach. Spare any change?
In contrast, the Los Angeles Skid Row district (with soup kitchens, shelters, etc) is located very far away from LA’s tourist attractions and other places for visitors. It is also segregated from the middle class, affluent, and struggling-hipster residential neighborhoods.
This leads to a difference of exposure.
With that said, I also agree with many of the other factors in the other answers here.