If you remember, the situation in Libya round about mid-march, when certain countries intervened, was that a city - Benghazi, was threatened with extermination by the forces of a gvernment which had not - and has not since - hesitated to massacre its own people, and which was involved in a civil war. At that time it was possible to aid the rebels by imposing a no-fly zone and neutering Gaddafi's air defences. While the Syrian govt has also been slaughtering its own people there is no fighting force to support, and no easy way to intervene as was done in Libya, without committing an invasion force. Plainly this would be madness and no one is contemplating doing it.