CNG's always been real popular for fleet vehicles because it's a lot easier for the fleet operator to use their existing natural gas infrastructure to refuel. So any benefit they'd get from electric they can also get from gas.

I briefly looked at the CNG Civics back when I lived someplace that had gas. The main disadvantage was half the trunk was taken up by the larger CNG tank since it's less dense than gasoline (which would be a non-issue for a bus).

The engines also last forever because natural gas burns so clean.