Reality, pure and simple. Or the idea that reality could be so interesting that someone would want to film it.
For example, why did so many people watch The Real World/Road Rules? The Osbournes and Gene Simmons? Or Growing up Gotti? Or Dog the bounty Hunter?
It's because while these people probably richer than us, or at least not likely to encounter us in real life, the idea that they're basically just everyday people makes us feel like our lives could be that interesting.
The Osbournes, when it comes right down to it, are just like any other family. A little dysfunctional, but then meet my family sometime and offer us a two year contract and see what happens. (They did it with John and Kate, so why not mine?)