Because it would cost hot gobs of money to patrol the entire transit system to the same level as City Hall? The tough on crime approach has been tried countless times and won't work. Adding turnstiles in transit stations, hiring hundreds of police officers, trying to teach troublemakers a "lesson" by throwing the book at them -- all expensive solutions that won't actually solve the problem.
I stand to be corrected, but my understanding is that there was private security at Coliseum station the day that lady was almost beaten to death. How many police officers are going to be required at stations and in trains to keep people safe? If we're closing in on 30% of the entire operational budget spent on police and we're still not getting the outcomes we want, maybe another approach should be considered?