Long ago, a part of the galaxy suffered a terrible plague that wiped out their food. The Watchers used their knowledge to develop a cure. Uatu used the virus to then turn all other Watchers into Virals leaving himself completely immune. He would use the beacons to send others to the Universal Library so the Virals could attain new and updated knowledge.
In the Comics
Uatu's father gave technology to a young civilization. When they returned they found that the civilization had developed nuclear weapons and wiped themselves out. This is why they force themselves never to interfere with other civilizations.
Uatu is not the last remaining Watcher. In fact, he is often punished by the others for interfering with Earth's events.