Population depletion is one factor. It was such an effective disease it could a) spread astonishingly quickly and b) kill incredibly quickly.

Once it killed off like90% of a population there are just far fewer people to walk around infectign eacxh other. We had also figured out cats and rats were linked to getting sick-via their fleas, so we had killed laods of them which limited the animal spread. There's some modern speculation the plague mayhave also been airborne for it to spread like it did but again, once there are far fewer people breathing bacteria all over each other, that limits that spread too.

It has an incubation period of between 2 and 6 days before you really start to show symptoms. In the peak periods when the plague it Europe and Britain we had growing and increasingly dense populations, and well established, well used trade routes.

That period, 2-6 days, it's perfect, it's quick enough for you getting sick to be a total surprise to those around you opening them up to infection, but it could be long enough for people who have no idea they're carrying this to travel out of a city to different towns, or sail to a different port, before they ever get sick and become an exposure risk. But then again, when you do fall ill you get sick so fast as to endanger people around you.

But once 90% of those people died off, survivors learned enough to stay awy from the sick or infected towns, to burn the dead and their belongings, we learned a lot of important stuff about limiting exposure and changes to living styles.The surviving population then had more resources when things recovered so could become healthier.
Plus plague is THAT deadly it's safe to assume many survivors would also actually be immune for one reason or another which obviously, impacts the spread.