The King couldn't express how happy he felt that Marcus was not guilty. But he couldn't set him free as he had no evidence to prove it.
The only way to get him out was with the queen's order. Reina was the victim and only her words can get him free. He could count of finding the midnighters and getting them to confess.
Those witches wouldn't dare free Marcus. Since they framed him, it meant that him being imprisoned was what they wanted and setting him free would ruin their plans and they would die before letting that happen.
The worst part of the situation was that Reina had changed her mind about setting Marcus free. She had taken her words back which meant that the man would have to go on trial.
The charges against him were heavy. With the queen mother and the princess hating Marcus he didnt trust them not to be biased towards him. All his queen wanted to be in his favour and that came at the cost of the one that was.