Hokutou no Ken was created in 1983. A lot of inspiration for the post-apocalyptic world in the manga came from the movies Mad Max (1979) and Mad Max 2 (1981).
In these movies (especially in Mad Max 2), oil is very important. Gangs of marauders fight for oil. They have the same kind of vehicles as we can see in Hokuto no Ken. In fact, in Mad Max 2, Max, the hero, protects a group of people around an oil refinery.
Ken and Max have a lot in common as characters.
So why is oil so important in the movie Mad Max in the first place?
In 1973 the world endured what was later called "the first oil crisis". 1979 was the time for the second oil crisis.
So the 1970s were years when people were talking about oil a lot. It was THE topic of the decade (maybe the second topic, just after disco music). In a way, it is logical to imagine that a movie made at that time would give such importance to finding oil.
If you have oil, you have power. In a post-apocalyptic world oil would be more important than money. In fact, money doesn't exist anymore, there is only barrels of oil to trade.
For the manga, the world is very similar than the world of Mad Max 2, but the topic of fighting for oil is not the main theme. Ken fighting for revenge, seeking his brothers, martial art, ... are the main themes. The problematic of oil/energy is not really explained, since it would be a little bit off-topic.
The enemy organizations are just bad guys and the don't really need a reason to attack the normal citizens. They attack normal people because they are bad guys. This is a reason good enough for Ken to fight them. There was no need to add some stuff about fighting for oil.
If you really need an answer, we can probably say that the bad guys are fighting for the control of oil. The gangs probably gather around an oil refinery, or a storage of oil barrels. This explains why the bad guys have vehicles, while normal people just live where they can. However, this problematic was not considered important to the scenarist and he just omitted it.