It didn't disappear.

There have been multiple smaller outbreaks since then, and one major pandemic in the mid-1800s.

The last plague oubreak in the US was in Los Angeles 1924-1925. Killed about 30 people.

The reason the plague ended was that people stopped spreading it. Successful implementation of quarantines is largely credited with depriving the disease of new human vectors. That, and it had already killed a third of Europe and basically burned through most of the susceptible population. It's harder for a pandemic to spread when the population density has dropped that much.

ETA: You can still catch a Yersinia pestis infection today if you really want to. Go hang around with the wrong rodents in, e.g., the US Southwest. A couple in Mongolia recently died of Plague after eating the wrong marmot.