Costume Drama, Courtroom Drama, Biographical Drama
Technically, almost every non-documentary movie involves costumes (since a costume is whatever the character happens to be wearing), which might render the term "Costume Drama" kinda confusing.
But when people say "Costume Drama" they really just mean it's a movie that's historical. A Man for All Seasons is just such a movie—take a gander at those puffy sleeves. This film deals with the conflict between a king and a Pope, and More's role in that conflict—a crucial moment in British history.
Also, you could argue that A Man for All Seasons is courtroom drama. Sure, More is only in a courtroom at the very end, but the actual interrogation of More and the legal proceedings against him all count too. These are legal matters. Plus, the whole plot of the movie is about a man trying to defend himself through the law while still being true to his principles. So A Man for All Seasons wouldn't be out of place if it were grouped in Netflix with A Few Good Men and The Verdict.
Finally, A Man for All Seasons is a biographical drama. We don't get to see young Thomas More refusing to sign some sort of tree house club oath or anything, but we do get a lot of biographical detail about the guy. It fits in well with other biographical movies about lonely heroes struggling to live life on their own terms—Bolt's script for Lawrence of Arabia would be another good example. (More could've easily dropped an album with the same title as 2Pac's Me Against the World.)