Interesting question. Typically, for a series to make it to syndication, you need a minimum of five seasons of episodes. You need a certain level of success during its first run. You need a distributor. And you need a plan to market the show to affiliates.
Babylon Five went 5 seasons, and was a moderate success. It was produced by Warner Brothers TV, and they do a ton of syndication. I gather the company felt it was more profitable to sell the series as videos direct to consumers rather than pursue syndication.
Battlestar Galactica didn't get the five seasons minimum. Universal had used the name and franchise for a number of platforms, from the TV series to films to games. They also are available as videos.
The one thing that is becoming a new platform for syndication, and it may be the way for Desperate Housewives, is On Demand through Hulu and Netflix. On Demand TV has potential for being more profitable than local syndication. It has the additional benefit of being low cost, and so not dependent on a certain number of episodes or attracting a large number of affiliated stations. I saw that Babylon Five is available on Hulu now.