Good question. No negotiating partner would put their best and final position forward knowing that the US Congress could come back and change the deal. This would result endless negotiations.
Right now, TiSA and T-TIP text are completely secretive and unavailable for even members of Congress to read while TPP text is available for members to review—although they need to go to a secret room inside the Capitol where only members of Congress and certain staffers high-level security clearances, who can only go when members are present, can read the bill.