The short answer to the question is, "Because the Obama administration doesn't know what the heck it is doing, has no plan, no objective or goal, and is generally screwing things up because that's what this administration generally does."
I have written repeatedly: there is no US national interest to be served in aiding the Syrian rebels. If they prevail they will be at least as hostile to the United States as Assad, and probably more so. They will furthermore carry out a ruthless bloodbath against Alawites, Christians and Druze.
- "How Obama bungled the Syrian revolution," Washington Post: "it is Syria that is Obama’s greatest failure."
- "About Those Syrian WMDs," Belmont Club