Other theory:

http://fincen.gov/statutes_regs/guidance/html/FIN-2013-G001.html

Under some interpretation, Coinlab would have to pay $15M to get the govt license. They don't have that kind of capital, at least officially.

As an established player in the North American Bitcoin industry—registered and fully compliant with the Department of the Treasury’s Financial Crimes Enforcement Network (FinCEN)— CoinLab provided Mt. Gox with U.S. financial and investment partnerships necessary to drive more U.S. volume and pave the way for institutional investors and high net-worth individuals to buy and hold large amounts of the digital currency.