It is apparent that there’s a split in the US ruling elites. Call it globalists versus nationalists.
The globalists saw overthrow of Assad government as more in line with their goals than defeating ISIS. ISIS was, alongside al-Nusra (aka al-Qaeda in Syria) the biggest threat to the Syrian State, so had to be allowed to grow at least until regime change accomplished.
LtGen Michael Flynn who was chief of the Defense Intelligence Agency at the Pentagon, in a very long interview on al-Jazeera, clearly stated the above to be true and said it was a “wilful decision of the [Obama] administration” to “allow the rise of ISIS”.
Flynn later became close adviser to Trump. Both would seem to represent a the nationalist faction in US elites. This is evidenced by Trump administration getting out of the globalist TPP, TTIP, NAFTA and the tariffs on China.
Trump’s election victory meant a change in policy in Syria. It was defeat ISIS because they’re bad and if they did take Damascus all hell would’ve broken loose. The globalists didn’t care about the Syrian people, nor about the cannon fodder of Western servicemen and women who’d have been thrown in to liberate Syria from ISIS and establish globalist ultimate control.
This split probably goes through the military as well. If you’ll recall there were media reports at the height of the Syrian war of CIA backed Islamist rebels fighting against Pentagon backed Kurdish SDF. So when Trump says he “knows more than the generals in Syria” it may be because he has classified info from the DIA.