According to BART it's due to insufficient train cars and outdated control systems. They say they're working on it.[1] Reading between the lines it sounds like

According to BART it's due to insufficient train cars and outdated control systems. They say they're working on it.[1] Reading between the lines it sounds like there is a money problem constraining it, and then we get to the controversial issue of how they manage costs. [1] https://www.bart.gov/guide/faq#frequent_trains https://www.bart.gov/guide/faq#frequent_trains