That is an excellent question, and while I can not answer for certain, I will speculate as to a few reasons why we do not use another type of fuel.

Modern cars are already designed and optimized for petroleum based fuel. Switching fuel might mean redesigning the entire engine, fuel supply, exhaust system, etc. The new fuel might not be compatible with currently owned vehicles, so switching would require everyone to replace their car, which would be very expensive. It might also require modifications to all gas stations for how to store and distribute the new fuel. Essentially switching fuel would require a large and expensive upset of the status quo.

While petroleum products are in limited supply, the shortage is not really significantly affecting us yet (yes, people complain about gas prices, but they still keep buying it), so there is very little economic incentive to switch. Gas and oil can still be produced very cheaply relative to other fuels, so people keep using them. Also, oil companies are still making huge profits and therefore will continue to promote cars that use petroleum products.

Another issue is: what fuel would we switch to? What is more abundant, cheaper to obtain, easier and safer to store, and at least as efficient, etc? It seems that biofuels and hydrogen are the two most likely candidates, but they are having trouble disrupting the dominant industry. Biofuels have the potential advantage of being compatible with already existing vehicles, but are still expensive to make and have their own set of environmental issues. Hydrogen fuel requires a complete re-design of cars and gas stations, not to mention we do not yet have a good way to cheaply mass produce it.

Getting away from oil is more difficult than simply finding a better fuel. That new fuel needs to be significantly better and cheaper and its use needs to be easy to implement, because displacing oil requires disrupting a well established system.