It's secret because it would be monumentally ineffective to negotiate a trade agreement in the open. Ratification or rejection, once negotiation completes, should be done in the open, but not the negotiation itself.

The reason is simple: the parties in a trade negotiation generally cannot get everything they want. They have to concede on some points in order to get what they want on other points. Over the course of negotiation, when a party is offering to conceded and what they are standing firm on changes.

You might have a party asking for, say, terms favorable to their automobile manufacturers but to get those they have to allow terms that are unfavorable to their clothing manufactures. As negotiations progress, they may be able to give up the automobile terms to get back the clothing terms and pick up things in agriculture and entertainment.

If all this were open, every one of those changes would be met with intense lobbying and political pressure domestically. The President, every Senator and Representative, every Governor, and probably the mayors of every major city, would be putting pressure on the negotiators to try to favor their biggest backers. The negotiators would be constantly being called to testify at hearings over each iteration. It would be a mess.