Why is the 5-Fulton line broken into sections?

Why can't the 5 Fulton line run from the Ferry Terminal to Ocean Beach only? It is very inconvenient to have to stop at 6th Avenue and wait for the next 5 to come around, especially on the weekends and in the evenings. It is also frustrating to get only as far as 7th Street during the abridged scheduled when trying to reach downtown. The location of that last stop is in an unsafe area, and as a female, it can be unnerving to either walk through the Tenderloin, or wait for a bus in the TL while getting harassed. Since the Richmond District does not have a light-rail line, many residents depend on this line to get across town. It deserves a more consistent schedule and route.