They chose to use that word on purpose, and primarily because it is meaningless in law. There was collusion, that is well demonstrated (see the Mueller report for one). Did that collusion constitute legal conspiracy? Mueller did not say so.
Collusion as intended is a moral-ethical concept and not a legal concept. Collusion with a foreign power to cause harm to the US is, at minimum, unethical and therefore inappropriate, and I would say immoral; a wrongful act. Whether it constitutes a tort, a criminal act, or treason is a matter of degree (effect of the act) rather than of fact (the act itself is not what is illegal per se).