In a poll, 40 percent of respondents said they're tired of reality TV, calling it the most overdone television genre. However, 15 of this summer's 20 highest-rated programs, including Jersey Shore, Big Brother, and America's Got Talent, were reality shows.
I think high viewership of reality TV can be lent some other factors as well, though, mainly time slots and ubiquity. Most people have the time to watch TV between, like, 8 and 10:30. If what's on in those hours, several times a week, is reality TV, then yeah, it's going to get higher ratings.
Also, people like watching shit shows more than they care to admit. It's like how everyone complains about cars slowing down to see accidents but just about everyone slows down when they see an accident.