Syria's army is too weak to contest all of Syria and the army has lost control of large swathes of territory:
Much of the army strength is tied up in bases that are basically under siege by rebels. The troops can only get out (or get supplies in) via a major military operation. These isolated bases will eventually fall as they run out of food and ammunition. The rebels, ever careful about their own casualties (an important element in maintaining morale), are content to wait the army out. Foreign reporters are able to travel near these besieged bases and see for themselves this aspect of the war. While the rebels can besiege government troops, the government cannot do the same to the rebels. Most of the countryside is under rebel control or a no-man's land. There are not enough loyal (to the government) police and soldiers to assert government control everywhere. Only in the largely Alawite areas along the coast is the countryside pro-government.
Using the air force to compensate for lack of ground power will only work for a while until the air force erodes away and can't fly:
The rebels believe that the only thing keeping the government forces in action is the air force, whose helicopters and bombers provide an edge in terms of reconnaissance and emergency firepower wherever the army is in big trouble. The big problem the air force has is fuel, which it is rapidly running out of. Aircraft consume tons of the stuff each time they fly off on a mission. While Syria stocked reserves of aircraft fuel for emergencies (like a war), these are not being replenished. As the warplanes fly less, the Assad forces will lose more ground and, eventually, everything.
By the time the West gets around to a no-fly zone, the Syrian air force will probably be incapable of flying anyway.
But use of air power buys time before Assad has to make another decision about the war. He's just buying time hoping something will save him. When that's all you can do, that's what you do.
But what will the next decision be? Will it be the use of chemical weapons to terrorize the rebels' civilian supporters? Will it be an attack on Israel, either directly across the Golan line or via Hezbollah and Hamas? Does he surround some bastion of rebel support and really inflict a mass slaughter in a short period of time in an effort to stun the rebels into submission?
Will Assad finally cut his losses and officially abandon portions of Syria to hold a rump Syria of some size--from a small Alawite enclave to possibly a larger realm that includes inland buffer territory and possibly the capital or even parts of northern Lebanon?
Will Russia step in to help defend the new and smaller rump Syria?
Does Assad decide to flee into exile?
Assad doesn't have many options. But he does have decisions to make. I don't know how long Assad can keep his planes in the air to fight the rebellion, but when he can't keep them in the air, he'll have to make the next decision in the war.
