I've addressed it several times, but I'll do it again. The reason NAFTA is not a simpler and more transparent treaty is in large part because of the attacks on it by anti-free traders of the right.
This was John Fonte's appraisal. I agreed with it:
The National Security Strategy rightly promotes free trade as an important foreign policy tool of the United States. It is important that both the World Trade Organization (WTO) and the North American Free Trade Association (NAFTA) remain instruments to liberalize trade among nations and not expand into areas properly left to democratic decision-making. As the American Enterprise Institute’s Claude Barfield has warned, the WTO needs
to ensure that its judicial bodies do not indulge in ‘legislating new rights and obligations through judicial interpretation’ that weaken democratic sovereignty
and has recommended measures to ‘rein in’ the WTO’s judges.
It's usually a correct assumption that if something is on the level and is inherently "good," transparency will be exist, regardless of any presupposed and/or assumed objections. Conversely, lack of transparency is a good indication the opposite exists, regardless of the excuse given for the lack of transparency.