Because what would replace Assad would even be worse.
DO i like Assad? No but all the major anti assad groups are radical muslims. The largest anti Assad force is Ahrar ash-Sham is basically diet al qaeda. They are aligned with nusra.
Zahran Alloush who was the leader of Army of Islam (until Russians killed him in December) has praised bin laden repeatedly for the 911 attacks. Army of Islam is the second largest anti assad faction besides nusra and isis
So that’s what you get with Assad gone.
Oh and lets look at the islamic front, another “moderate” anti assad faction, only problem is that they are also diet al qaeda, they are a saudi proxy that promotes radical wahbisism