The head of HHS should have a history of good judgement.

Credibility and risk of relapse are two reasons that quickly come to mind.

Also the fact that he went from heroin addiction to using steroids to enhance his sense of his own masculinity.

And HGH, either/both of which he is obviously still using.

As I noted above, addiction in adolescence while a human brain is still developing (which continues deep into our 20s) forever damages its architecture and prevents users from maturing in important ways. I read a lot on this topic during the Kavanaugh confirmation hearings.