Article 25 of the Universal Declaration of Human Rights
Everyone has the right to a standard of living adequate for the health and well-being of himself and of his family, including food, clothing, housing and medical care and necessary social services, and the right to security in the event of unemployment, sickness, disability, widowhood, old age or other lack of livelihood in circumstances beyond his control.
I see three possible reasons:
He's dead
El Salvador and the US don't want anyone to speak about what's going on in the prison
Trump wants to use this as a test. If he can get away with this, he can get away with sending most anyone.