They think that Assad's immediate departure would lead to a collapse of the regime. Washington also sees a rapid collapse of the regime as something that would be a boon for ISIS. They are in a conundrum:
if Assad goes right away, it would help ISIS, but if he doesn't go at all, you have no hope of putting the pieces of Syria back together again, this recent outburst of diplomacy is because everyone was becoming concerned, and rightfully so. But the results of that process were remarkably poor. They seem to cement the earlier political positions of the region when it comes to Syria.