By "gas", I assume you mean compressed natural gas, or propane, not "gasoline".

The quick answer is that you CAN convert your car in the US to run on LP gas. Google it.

It's not "mainstream" because people like the convenience of getting gasoline (or diesel) at any ol' corner gas station. If you go LP, you've got to go to an LP filling station, which is not nearly as convenient. In the US, it's more suited to fleet vehicles, where the fleet owner can maintain their own LP filling station.