Other mistake: After Frank releases Dr. Scott from the Triple Contact Electro Magnet, Janet and Rocky are heard giggling in Rocky's birthtank. As Frank walks over to the tank, the right magnet pops out again just before it goes off-screen. (01:02:45)
Other mistake: When the Narrator looks at the photographs in "The Denton Affair" album, some of the photos don't match up with what actually occurred in the previous scene. Things such as, when the car pulls up, and the driver exits and moves out of view, there are only a few people left on the porch, but in the photo there is a crowd on the porch.
Other mistake: When Frank is awakening Rocky he tells Riff Raff to turn up the power three more points; the power hadn't been turned up in the first place.
Answer: It has never been expressly stated, though many theories exist, the primary one being that Frank and the others were sent to Earth simply to experiment on human life and possibly reproduce it (Frank is a scientist, they have a lab, he creates Rocky etc.) and to test their technology on humans. This would require a level of covertness, which Frank disregards by flaunting his hedonistic lifestyle, and thus could have been seen as a failure.
Purple_Girl