It's not because immigration became harder - they are all MScs or PhDs, some even already did internships in the US, so moving shouldn't be hard. It's because we think that the US is no longer a desirable place to move to.
It's been kind of hard for me, because my visa process was impeded by USCIS. My compay has moved people to US before, under Obama presidency, and the process was much easier back then. And you're right, US is no longer a desirable place to move to, now that Biden has won.