The correct answer is because the term involves a stereotype.
The term minority is considered offensive by critics because the word minority is often associated with stereotypes that are exaggerations or entirely untrue. Minority connotes that a person is different, inferior, at a weaker position and not "normal" to an extent. For instance, if you hear something about racial minorities, its unlikely to be something positive about that race, and in most cases it will something that reinforces the stereotype of a person belonging to a racial minority.