Sam-myeong doesn't have the power the get him out of there, and the King didn't want him leaving the premises either. It was stated very clearly that the king thought Chang-geon capable of overthrowing the first and second prince since he had the intelligence and the motives. It's heavily implied that the king feared Chang-geon might scheme if he left with Sam-Myeong. Like already, he was learning quick and reading lots with little to no resources. Sam-myeong's marriage was essentially a punishment and warning to not return.

can someone remind me why the uke left the seme there in the first place? I know he left because of marriage, but he knew that ml was being abused there by his family! I doesn't make sense to me