There are two main competing theories (well one is by me and the other is by my friend). It is important to note that the two theories are not mutually exclusive and the effects might combine to interesting effects.
Generational Aging Theory
All the friend watchers grew up. They have moved out of the single fun stage of their lives, and now own houses and have kids in the suburbs. Instead of wanting to see people who have perfect lives in the big city, the grown up friend watchers want validation about the choices they have made in their lives. In other words, they want to see how their lives could be worse. Housewives provides that.
Sex Sales Theory
A friend said that the above theory was not the explanation, but that "Horny Guys" were. Speaking as a horny guy, I am not really interested. The name is as catching as a spam email (which is pretty catching to think of it), but when you actually watch the show you figure out that you are the one getting screwed. The women look great, I cannot deny that, but they spend a lot of time talking about their relationship problems, instead of getting it on. In the show I saw, terri hatcher did get nekkid, but you did not see anything. I guess that only goes to prove that free porn is bad porn. She also is not as good looking as you remember, she seems like she has aged a bit.