There are a lot of reasons behind this, and a lot of moving parts keeping such deals from happening readily, but the main driver is a fear of the “nightmare scenario” of the Assad government collapsing outright and ISIS sweeping in to take the rest of a country they already control over 50% of.
Though the Assad government denies anything of the sort is imminent, the world can’t help but notice that its forces are increasingly on the defensive, and losing ground to ISIS, al-Qaeda, and pretty much everyone else challenging them. While still backed by Iran, Russia, and Hezbollah, the Assad government is struggling to keep its military going as the civil war continues to turn against them.
US officials have previously conceded that their goal in Syria was not necessarily a military-imposed regime change, but to keep the civil war stalemated so long that they could come in and negotiate a resolution that was more favorable to American interests. Clearly, that’s not ISIS taking over outright, and the effort to keep the combatants in stalemate isn’t working so well anymore.
Russia, by contrast, wanted Assad to win and remain in power, as their nation’s main Mediterranean ally. They too were averse to too much change, fearing a shift toward some other nation’s sphere of influence, but with the war going badly for Assad are similarly looking to avoid an ISIS win.
Other powers all have their own agendas. Iran wants to maintain Assad’s rule as a key ally as well, and Saudi Arabia, which desperately wants Assad out to spite Iran, is coming to the table mainly for fear of being cut out of a deal that happens.