US is not really to blame directly in this case. The US by now is willing to accept Assad for a limited time. Russia has also shifted and might accept that Assad has to go in the middle to long term. Their is a chance that they could agree.

However local powers, Turks, Saudis, Iranians are not that flexible. Iran really needs Assad in power, they really want to crush ISIS. The Turks don't want the Kurds to form their own country. The Saudis want Assad gone and don't want ISIS completely destroyed.