Well look at at this way. Assad has basically two options now and he fully understands the consequences of each choice.
Choice 1) Go ahead use those Chemical weapons. See how much longer you can?
Choice 2) Stop using them, stay in power a while longer and hope for some sort of negotiated power sharing with the rebel factions, or at the very least face the Syrian people in a vote to see who they prefer to lead them.
Both of course have risks to his power, but neither is without consequence. It's up to him to decide. He doesn't have any other choices.
Power sharing? Assad is close to regaining complete control of the country. The rebellion is failing and can't sustain itself much longer regardless. We have launched two strikes against the regime and both were little more than cosmetic. We called to warn the Russians each time, and they simply moved anything important to somewhere else and watched our missiles blow up valueless targets. Calling them was probably less of a warning and more of asking permission.
The rebels will be defeated and Assad will remain in power with the backing of Russia and Iran. The Kurds might be able to carve out a semi-autonomous region in the North-Eastern portion of the country , but they don't pose a real threat to the regime.
Why did Assad launch the chemical attack a couple weeks ago? It was because he was able to force dug in resistance to evacuate the area and knew that any response to it would be insignificant. Hell, the next day he was able to claim that they defended an attack from the United States, and the people living there believed it because nothing much had happened.