As I understand it, Tether was meant to be supported at a rate of $1 per Tether, with the dollars stored in Tethers vaults. Tether was meant to have regular audits to provide transparency and verification to its users. However, the last major audit was fraught with problems and the auditing firm quit without publishing any reasons as to why.
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.
However, it has been over a month and there have been no new developments, so no-one is really sure what is going on there.
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.