A Quick History Lesson : Why The N Turns at Irving and 9th In the First Place...
Several people have asked me why it is the N-Judah makes the infamous left turn onto 9th street in the first place, and doesn't just continue on Irving, OR stay on Judah/Parnassus. Aside from the obvious (it wouldn't be the N-Judah if it was on Irving, it'd be the N-Irving), the reason you have this detour is because it's a relic from the olden days of multiple service providers in San Francisco running rail lines.
Quite awhile back we had a photo of the old 6 Parnassus streetcar and the N on Carl Street. MUNI and the private operators avoided sharing rails whenever possible, hence the split we have to this day, even though the 6 Parnassus ceased to be a rail line decades ago.
Many of the bus routes of today are leftovers from when the Market Street Railway merged with MUNI, and from other smaller providers of transit service, when various companies would attempt to duplicate service. And yet if you try to change anything there's usually someone demanding it all stay the same. That's why the Transit Effectiveness Project is trying to analyze how people actually use and don't use the system, so they can revise it to make things run for today's needs, not for 20+ years ago's needs. Hopefully.