Chad is one of those countries where things like this are possible because, outside the capital (and key
President Déby is certainly no democrat. Technically, he's "elected" against "opponents," but he did have the minor incumbency advantage of coming to power in a rebellion in 1990 which ousted his former boss, Hissène Habré, who ruled from 1982-1990. Chad has a long and storied history of aides and deputies deposing their previous employers, which also appears to be the case with our current rebel, Mahamat Nouri, who apparently has been a top gun for both presidents. (While I'm not a Chad expert, I do get the sense that a large number of people in the narrative change sides frequently in an opportunistic fashion). Nouri heads the "Union of Forces for Democracy and Development." This name is undoubtedly accurate insofar as it is (maybe) a union of some type of forces.
Déby, by the way, is hated by most people after changing the constitution to allow himself yet another term in office in 2006--not that he was Mr. Popular before then, having stuffed the army with members of his small Zaghawa group. He's also the one who gave the finger to the World Bank, taking the money out of that account which was supposed to be for "long-term development" use and spending it (surprise!) on the military.
Now, there is no reason to believe that Nouri would be any different than Déby, at least that I can figure out. This makes reason #1 that the French continue to back Déby. Better the devil you know, right? Reason #2 would be the Sudanese support for the rebels; no need to make Sudan feel more secure what with everything going on in Darfur. Reason #3 would be the danger of creating more instability, not just in Sudan but also in the neighboring Central African Republic, which, if it were a pigs' house, would definitively be of straw. It will be interesting to see what develops in the next 48-72 hours with regard to the situation.