The problem is that if you negotiate in public, pressure groups have the ability to influence or sabotage and sabotage the deal before it's even complete. This

The problem is that if you negotiate in public, pressure groups have the ability to influence or sabotage and sabotage the deal before it's even complete. This can happen even if the the deal would overall be good for the public. Keeping the negotiation process secret allows it to be analyzed as a package.