A massive barrier to this is that people generally seem unhappy with the fact that public transit means they need to sit next to other members of the public. Rightly so, to a degree - our fellow humans can be quite rude or unsightly or dangerous at times, and most of us have probably done some shitty things in public by accident a few times. So if you offer them a pipe-dream of private automated transit instead, that's a lot more appealing than dropping 8 figures on slight improvements to BART or their local buses.

The area I grew up in had a light rail network that was supposed to expand to my community college within 3 years, and in practice it took over 10. One source of delay was that communities rejected the expansion, and one reason among others was a claim that Local Crime would 'ride the rails' to their previously quiet neighborhoods. Sometimes people don't want to share space with their neighbors.