With the US leaving there isn't anyone left to keep the Syrian Army from crossing the Euphrates and finishing the job
Trump has said he is withdrawing US forces from Syria because ISIS has been defeated there. That isn't actually true. A small ISIS enclave has survived on the left (eastern) bank of the Euphrates river near the border with Iraq for over a year now.*
This is a feat the ISIS enclave has exactly US forces to thank for.
On the one hand the US military warned the Syrian army against crossing over the river to finish off the ISIS there, and on multiple occasions backed its threats with force when the Syrians attempted to do so regardless. The US even rattled sabers when the Russians flew bombing sorties against ISIS in the area in support of the US-backed Kurdish fighters.
On the other hand the Kurds the US had doing its bleeding and taking of territory from ISIS to deny it to Assad has been unwilling or unable to finish off this last ISIS stronghold. This isn't surprising. They, not the Americans, are the ones expected to do all the bleeding, and for Arab-populated settlements. Moreover they had Turks breathing down their necks in the north and the Americans didn't lift a finger in aid when the Turks invaded and overrun the separate Kurdish-held Afrin enclave in the northwest. They were the recipients of US arms, training and money but they also understood that was only sure to continue while they were still useful to Uncle Sam's fight against ISIS. In those circumstances nobody would be in a hurry to finish the job.
Moreover knowledgeable commentators have speculated out that perhaps the Pentagon itself was in no hurry, for the exact same reason. While ISIS still held some ground it could defend its continued presence in Syria to the White House and the American public.
Trump cut the Gordian Knot by ordering a withdrawal anyway, which is absolutely the correct move to take politically and morally, but it does show what a hodgepodge his leadership has been. If he intended to withdraw he should have never allowed the Pentagon to block the Syrian army in such a way in the first place.
Because he did not, he is now withdrawing in circumstances where establishment opponents out for his blood are indeed technically correct when they point out that he is not telling the truth and that ISIS has not yet been finished off. The American public will likely not care a great deal (they'll be busy celebrating that they're out of another confusing Middle East war), but he need not have been vulnerable on this at all -- if only he had let the Syrians and the Russians grind ISIS into dust way back in 2017 as they wanted to do.
*Last week ISIS was pushed out of the town of Hajin which was the enclave's largest settlement, however the outfit remains in control of a whole stretch of farming settlements to the south of it, all along the river, nearly to the Iraqi border.