The Sneepers are a race far older and initially more advanced than humanity. They developed space-faring technology early on and used it to invade other planets. They conquered a number of unnamed races. While scanning for other life in the universe, they discovered Earth. It was too far from their planets to be reached easily. but they decided to keep it under surveillance anyway.
When initially discovered by the Sneepers, humanity was technologically and culturally primitive. The cave men were deemed beneath the Sneeper's consideration. The rise of "Mighty Empires", particularly the Roman one, alerted them to human capacity for advancement. The Renaissance alerted them to the existence of human creativity on multiple fields. The Industrial Revolution had them marveling at the speed of human progress.
Since they progressed at slower rates, the Sneepers started worrying that humanity might wrest control of the galaxy from them. When humans discovered airplanes, the Sneepers figured rocket ships would be the next step. They started developing super weapons to be ready for a possible military conflict with Earth. But then Earth's nuclear technology surpassed their own methods of energy production.
Waiting for humanity's next step, they were overjoyed to see humans threaten each other with their nuclear weapons. By the 1960s, the arms race, space race, conflicts in Cuba and Vietnam, rising crime rates, riots and strikes managed to convince them of one thing. It was only a matter of time for humanity to destroy itself. They decided to stop any actions against Earth, waiting for humanity's destined self-destruction. Uatu the Watcher adviced us against doing the Sneeper's work for them.
The Sneepers were created by brothers Stan Lee and Larry Lieber in "The Saga of The Sneepers" in 1964.