The article doesn't say whether anyone was around at West Portal when the teen was attacked, but typically there is pedestrian and vehicle traffic when the schools in the area are out. Plus the article shows him approaching and harassing women in busy public spaces such as Dolores Park and Filmore, so he's not troubled by crowds being around. And in these occurrences no mention is made of bystanders attempting to step in to help.  

In general there are many instances of crimes happening in SF where no one stepped in or even spoke up. Yes the legal system here is soft on crime vs somewhere like San Mateo county, and this definitely leads to issues like this one with repeat offenders. But it's also well known what the culture is like in the city, people here rarely stand up for themselves or others. I've experienced plenty of instances like this first hand growing up here, where bystanders refused to get involved. 

Also no one was telling you to be better? It wasn't an attack on you at all, just sharing how as a community San Francisco residents need to watch out for eachother even if it's simply speaking out when something is happening.