I think the people buying these are worried that the cash won't exist in 90 days. If the bank they deposit their cash in goes bankrupt they get nothing. The people buying the T-Bills are well above the $100k FDIC limit. Therefore, by buying the T-Bill, they protect their downside by betting the US government does not go broke in the next 90 days.

That would be my guess as to why they want to stash their money away for 90 days. After 3 months the buyer can find out which bank still exists to even take a cash deposit. Cash in the T-Bill and decide what to do. In the meantime, buyers have 3 months to figure out what to do in this market. Its almost like hitting a pause button with your money and giving yourself time to think this out.

ps. Not a finance guy here by any means.