Lack of resources. Difficulty of movement. Survival.

Consequences of the alien impact. All of this has drastically altered the familiar order of things. Conventional technologies have given way to new, strange ones. Instead of car engines, people now use artificial pig hearts, and the fuel comes from the liquid extracted from the head of an incubus. The production and transportation of these hearts is incredibly expensive, and only a few can afford a vehicle powered by a legally-produced pig heart. There’s an alternative — the black market. Illegal farmers grow real pig hearts inside living pigs. These hearts are short-lived and tend to break down often, but they cost a fraction of the legal ones. Despite the efforts of official producers and local authorities, the black market continues to thrive. Detectives often either own such farms themselves or "cover" for illegal producers, doing so quite openly. When people are desperate for resources, they couldn’t care less about the law.