Aren’t most busses CNG these days?
CNG and propane have much better emissions profiles, and vehicle lifetime and compressed tank lifetime are a good match for transit, as opposed to personal vehicles where when the compressed fuel tank ages out, the otherwise servicable vehicle turns into a pumpkin.
However, CNG ends up being expensive and may not save much versus diesel... The natural gas is usually not expensive, but compression requires a lot of energy input which is expensive.