14Humans guarded and inhabited it for centuries
The people of Gondor weren’t about to take any chances just because it looked like Sauron was gone. They still had a place full-up with orcs and monsters on their border, so they immediately took steps to keep it contained.
They built a number of fortifications to monitor and fight off
any evil things that might try to get out, and this plan worked pretty well for over a millennium and a half. However, a plague broke out that devastated Gondor’s population, and the garrisons guarding Mordor retreated to protect their cities.
Once the plague had passed two centuries later, the population had recovered enough to return to Mordor and continue the watch. However, by then, it was too late to deal with what had happened in the meantime.