If I understand the issue correctly they fall into different visa categories. Postdocs seem to be able to get J1 visas whereas a lab manager would be H1B, which is the category that Trump's cracked down on to decrease immigration. I've heard even the likes of Facebook are struggling to get that type and there are I think costs for the employer, so not worth it to employ a foreign applicant when there are qualified US citizens. Similar to the situation in the UK really, so we can't complain.