Because doing things in public requires educating the populace on issues they don't understand.
Because doing things in public politicizes the process.
Because doing things in public leads to escalation.
Because doing things in private is how almost all conflict resolution happens.
But doing things in private increases voter apathy.
But doing things in private is suspicious behaviour in a Western democracy.
But doing things in private means that the people most affected will not be able to contribute to close loopholes, tighten up wording, spot inconsistencies, improve operational processes etc.
But doing things in private means that bribes, extortion, blackmail etc. are par for the course during negotiations, with no penalty of a backlash.
But doing things in private means she with the biggest bribe wins.
But doing things in private is merely a delaying tactic as the law has to eventually become public knowledge at which point it will be politicized, escalated etc.
But doing things in public is how we (attempt to) improve fairness in a modern Western democracy, despite the cost in efficiency.
Clearly there is a compromise to be made between the two extremes, as dlss's points are very valid and are constantly a challenge for democracies to overcome. But I posit that the balance doesn't lie on the "keep everything secret" side.
This is not true--as a treaty, TPP will need to be approved by the U.S. Congress to attain the force of law. It will be published and roundly debated at that time. If necessary it will be rejected or adjusted.
For now, negotiators are working in private so that they can work things out without several million backseat drivers.
This is how all international treaties are negotiated, by the way, including recently passed FTAs with Colombia and South Korea. People who make a big deal about the fact that TPP is being negotiated in private simply demonstrate that they are not familiar with international negotiations in general.
Because educating the public is difficult and takes time.
Because crafting the rules by which people are governed is an inherently political process.
Because deep divisions, and the theft of property rights from the majority by secret laws is an escalation.
Because doing things in a transparently public process is how you build institutions for the long term.
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If we are building a global government, let us build it with the long term benefit of humanity as it's guiding principle. Not with the short term profits of a tiny elite at the expense of the future of our species.
Any attempt to use trade negotiations to bypass national democracies is prima facie evidence of malfeasance. But further entrenching a shortsighted corporate feudalism that disregards ecosystem services and deprioritizes the health of the biosphere is species suicide in the making.