Edward Rees goes over logistics:
The Bosnian [No Fly Zone (NFZ)] was conducted over a small airspace just 51,000 square miles, and even the Iraq cases were larger at approximately 100,000 square miles. A proposed NFZ over Darfur was mooted in part because the region, at 550,000 square miles, is simply too big.
Unlike in Iraq and Bosnia, there are no obvious air bases near Libya from which to impose an NFZ, so the aircraft would likely have to be based on aircraft carrier. But it's not clear that the U.S., or even NATO, has that kind of capacity to spare. Using European bases would require many more aircraft as the greater distance severely limits the amount of time each plane could actually spend over Libya.