Why? Because Russia. As long as it remains convenient for Russia to use Syria as a distraction, Syria will continue to remain in turmoil.
Except, we know for a fact that Assad had chemical weapons, and there is plenty of evidence that chemical weapons were used, and Assad is the only one with the capability and technical know how to produce, develop, and deploy such weapons in Syria.
This isn't some great conspiracy theory. Keep in mind, the US wanted to STAY OUT of dealing with Syria, which is how Russia got so easily involved.
FYI, Iraq wasn't blamed for having chemical weapons, it was accused of having nuclear wmds. Iraqi chemical weapons were tolerated, nothing more.