All we can do is speculate, but my thinking is that Friedman dissolved the relationship for the same reason that Tether can't move money. It's probably not that they're insolvent. Tether's banks probably don't even know what the money is for, and resuming withdrawals en masse probably means getting accounts seized by regulators -- like the CFTC, who sent subpoenas to Tether 2 months ago. If so, Friedman had no way to verify Tether's obligations and ability to repay outstanding USDT.
The feds take years to build cases sometimes. Consider BTC-e. They unsealed the indictment in July 2017, but filed it 6 months earlier. They were probably investigating for years before that.