Well, it's also true that Gendarmes are sort of more efficient because they are less prone to the civilian regulations. Especially regarding the use of force and firepower. Even if, we are in France and Western Europe, it's severly and often stupidly limited. As for G.I.G.N./RAID, the first group is said to be more efficient too. Well, they are military.
But indeed, We are kinda satisfied with what we have and last decades attempt to make some fusion between police forces as well as to put an end to the gendarmerie failed. When you look at other examples (yes, Belgium, I am looking at you...), you clearly understand why. Still, we are (sadly) an unitary state, you are a federation. If I were a U.S. citizen, I would be a staunch supporter of the Posse Comitatus Act. The lesser the power of the central government, the better.