Honestly the bus network in LA is great as far as US cities are concerned. Great coverage, but hampered by sharing the grade with cars and waiting at red lights. All it would take is a can of paint for a bus lane and the same signal preemption enjoyed by police cars looking to run a red. No new technology is required or great investment beyond that.

Those largely died out because they were run by private operators, but the government then decided to make them cheaper by imposing price controls, and then didn't "nationalize" them when they ran out of business and died.

(The common story is they were bought up and shut down by car companies, but that's a myth afaik.)