Maybe because places that don't have a problem with crime in the first place don't care about making laws to restrict access to weapons?
Perhaps. Assuming you're right, it leaves open the question of whether the restrictions actually affect the level of violence and in what direction. The assumption of the cities with high violence and tight restrictions is that the restrictions reduce violence. Though that assumption seems logical, history calls it into question. For example, both DC and Chicago saw massive increases in violence after they enacted their draconian restrictions. The rest of the country also saw rising violence at the same time, but nowhere near in the same degree. So perhaps the city leaders were prescient, saw the coming wave of violence and acted to mitigate it, or perhaps their action actually exacerbated it. Or maybe the restrictions made no difference at all.
My money is on restrictions increasing the violence, mainly because the restrictions only affect the law-abiding, which gives criminals an advantage, and eliminates their single biggest worry (per FBI studies, in which violent criminals overwhelmingly report that their biggest fear when committing a crime is that the target might be armed). We'll get a chance to see over the next few years, since DC and Chicago have been forced by the courts to loosen their restrictions dramatically.