Trying to look it up, it seems like the reservoirs for it are rodent species that don’t just almost immediately die from the infection which happen to be ones that thrive in drier, warmer climates (although not exclusively: outside the US the main concentrations of reservoirs seem to be in the DRC, Madagascar, and Peru), while the kinds of rodents that thrive in urban areas are themselves way more vulnerable to it and die too fast to be reservoirs (this would be why the big historic plagues burnt themselves out relatively quickly: they wiped out their own vector enough that they stalled out and stopped rather than becoming endemic).

Since someone else mentioned prairie dogs, I’ll add that it’s apparently a big problem for those since they’re in the same range as its reservoirs but aren’t resistant to it so it can wipe out their colonies if they start to suffer an outbreak of it.