Why is it classified to begin with?
Because they don’t want US to see it.
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Why is it classified to begin with?
Because they don’t want us to know what’s in it. And they know we wouldn’t agree to it if we did.
That answer is classified.
In all seriousness, their argument is that if the details were released now, special interest groups could pressure the negotiators to make changes that otherwise wouldn't be made.
There is some truth in that. But it does not trump the right of the people to be informed! This is not planning for D-Day, after all.
Furthermore, if the negotiators are so easily pressured, then they are not very good negotiators in the first place.