BART works because it is a
heavy (not light) electric rail system with a lot of capacity that was built in the Sixties by engineers who knew what they were doing. The Bay Area - especially the East Bay - evolved around it. Trying to build such a system into an existing urban infrastructure like Seattle would be nearly impossible. BART couldn't be replicated today for less than 200 billion dollars and twenty years of work - extension costs now are running about a billion dollars a mile.
And the trains are dirty (largely because they let bums ride for free) and the system still can't turn a profit, despite very heavy usage.