When you consider that the plague came over on rats, it’s surprising that it doesn’t seem to exist in east coast ports, which still have plenty of rats, which surely have fleas.
On the other hand, it seems to have arisen in the steppes of Asia, so maybe the desert is more its home, in its low virulence form.
I'm given to understand that when the Great Plague hit Europe, it was during a time of suddenly-colder-and-wetter weather.
(as an aside, apparently induced by a sudden dearth of sunspots)