A trade agreement negotiation is adversarial. Each country is trying to get terms favorable to its citizens, and doing that will usually require concessions to the other parties.
A country that works out its plans for the negotiation publicly is going to be giving those adversaries a lot information that will result in the country getting a less favorable deal.
A country that works out its plans for the negotiation publicly is going to be giving those adversaries a lot information that will result in the country getting a less favorable deal.