Not true that they were banned. It was suggested - never collectively bargained and agreed upon until later.
In 1991, Commissioner
Fay Vincent sent a memo to all teams stating that steroid use was against the rules, though there was no official rule change. Vincent has said that the memo was intended as a "moral statement" to the players, rather than a "legal one", that "the only way a change could be made was through collective bargaining," and "When I left baseball, there was no written policy on drug activity in baseball."
Steroids were banned in MLB in 2005, HGH in 2011. Fay Vincent is on record of saying that he in no way banned steroids from MLB, but merely passed along the information that Congress considered the substances illegal without a prescription.