This article talks about the US as monolithic, but my limited understanding is that although SDF's key ally was the US Army, CIA had clandestinely been backing a different Syrian faction, so "the US" has never wholeheartedly been onside throughout. (and, presumably, now it's State's turn to be backing Erdogan? Pawns will be sacrificed.)
'America has no permanent friends or enemies, only interests' — HAK
The West just wants to hurt Assad and by extension Russia and Iran. It is truly an evil war. Syria would have been better off under Assad than with what has happened due to Western intervention. The bar for toppling a sovereign order should be very high, and should almost always include a high chance of quick success and a viable alternative regime.
Here's an article from a few days ago: https://www.middleeasteye.net/opinion/syria-turkey-kurds-roj...
It's disheartening but I guess predictable that a society which seems to be committed to democratic reforms in the Middle East is the one getting thrown under the bus.