Students don't like being told they're bad at something. Students don't like being told to do the thing they're bad at. They especially don't like having to constantly repeat this in a loop until they've caught up with the rest of the class, while other students get to enjoy their free time.

Also, mastery learning doesn't look all that effective on standardized tests: https://www.jstor.org/stable/1170613

How do you do Mastership in a class environment? Do they leave half the class waiting for everyone else to catch up?