Nico Harrison hid this trade from everyone because he knew it was malpractice and there would be public outrage from players, coaches, people, and the organization.

A lot of people keep asking why they did not reach out to other teams for more assets, and other things like that. And its still puzzling, but I think a major reason why Nico Harrison did not reach out to other teams and why this trade was done in full secrecy without even the coaches, players or agents knowing is because there would have been huge public outrage from Doncic, other players, teams, coaches, and people. It would have been so bad and everyone would disapprove it so much to the point where this trade probably would not have been able to go through. It would have been rejected by people within the Mavs organization and basically the public. Nico Harrison knew that this trade was a malpractice, and thus decided it was best to just not tell anyone even the coach or Luka Doncic. And IMO him operating behind everyone's backs without even telling the players' agents is very wrong on his part.