gasoline is that is made from a type of petroleum not common in the US (light sweet crude) and does not burn clean enough even with current high tech and costly fuel systems. Unlike an engine that runs on gaseous fuels such as CNG (compressed natural gas) or LP (liquid propane) and others you can breathe the exhaust with exception of the oxygen depletion as determined by EPA air quality standards. NO gasoline engine regardless of any add on high tech systems can come near to making this claim (unless the gasoline is converted into a true gas prior to combustion).
we have a finite supply of petroleum
it is a pollutant
Some people have a car that runs on some other form of energy, so it's cheaper to keep buying that form of energy than to buy a new vehicle that runs on petrol.