Discussions in the UN about issues as big as imposing sanctions or going to war have taken place with more transparency than we're seeing here. National politicians who have defied public opinion on such matters have sometimes paid a heavy price for it at the next elections.
I mean, seriously, we're talking about a bilateral trade deal here, not the fate of humanity. Yes, each party has some internal political structure and local variations, but the point of both the EU and the US is to have regulated, harmonious local variations so the whole can still function effectively.
Please tell me, if everything is above board, what is so secret about this kind of a deal that it justifies obstructing not only the public but their elected representatives from watching it develop? For that matter, why should those elected representatives not then block the entire thing on principle, if they are only to be asked to rubber stamp the results without substantial scrutiny or a meaningful opportunity to actually represent the interests of their electorates?