In case you haven’t noticed, when their is a problem on a bus line, the buses CAN be rerouted. Every day you see numerous notices from SFMTA about re-routed bus lines due to emergency conditions such as fires – or other alterations caused by heavy construction or roadwork.
Almost daily you also see notices about stopped trains and cable car lines –
SF Muni California Cable Car Line delayed due to an Accident. There is no way to re-route a rail or cable car so the whole line must come to a halt when such an incident occurs.
If the lower costs of purchasing, and operating a bus line were not enough to convince you that buses make the most financial sense, the reliability factor should be considered as well. That is why many professional transportation professionals favor buses over rail. You need a certain number of buses just to cover for the downtime of rail.
THE ECONOMIST: “Streetcars and Urban Renewal: Rolling Blunder”, “Federal subsidies have inspired some silly transit projects”, August 9, 2014: http://www.economist.com/news/united-states/21611123-federal-subsidies-have-inspired-some-silly-transit-projects-rolling-blunder