The creators of "Cheers" spoke to The Hollywood Reporter in 2018 and revealed that the star of NBC's "The Cosby Show" would have been their lead bartender, if not for two rules.
Creator Les Charles shared these two rules: "No known names and no [character's] name as the title of the show." Plus, Cosby was just wrong for the role in terms of acting type.
Years later, it was discovered that he would've been disastrous to cast for truly heinous reasons, which included drugging women's drinks and sexually assaulting them.
In the wake of his conviction, casting him as a bartender would've been an abomination. It would have meant the complete syndication death of "Cheers."