> What I don’t understand is why they couldn’t write a clear email explaining their position so you would actually know what’s up.

In some cases, it’s forbidden by law to reveal to someone that they’ve been flagged for money laundering.

https://www.law.cornell.edu/cfr/text/31/1020.320

> No bank, and no director, officer, employee, or agent of any bank, shall disclose a SAR or any information that would reveal the existence of a SAR.