I'm like 90% sure that the reason US backs YPG is because of the, ahem, ideological alignment of people staffing the American government at the moment. Not really a surprise that support for them started under Obama, Trump tried multiple times to get rid of them and got blocked by others in the administration and then Biden jumps in and surprise, surprise, support for YPG is back on. That being said even US officials are starting to recognize that backing somebody so useless that they performed worse against ISIS than flipping Hezbollah might be a losing proposition so I'd wager YPG's days are numbered regardless.