Vehicle

Vehicles, especially those running on petrol, are generally rare in the Fallout world. The most common means of transport are brahmin carts. The super mutant army is also said to have used some steam-powered trucks.

History
The Great War was fought mainly for resources. The last resources remaining were probably depleted in the war itself. Without resources, vehicles couldn't be driven anymore. After the war, a lot of human knowledge was lost. People forgot how to tend to the high tech pre-war cars and their complicated nuclear fusion motors. Most books were damaged beyond all legibility by the war and many computers were destroyed due to years of neglect, and with them the documentation for these pre-war vehicles was lost. This combined with the lack of large scale primary and secondary industry meant most vehicle production/repair was largely impossible.

This does not mean that there are no old vehicles that have been kept funtional or that someone out in the wastes could not have invented/innovated their own vehicles, but such instances are extremely rare outside of technologically advanced groups (i.e. groups that have overcome the struggle for basic survival and can put resources towards researching old/new technologies).