I assume the discussion in question IS a graded discussion?
Makes sense if it is not graded.
But perhaps it is because graded discussions may not be given grades until the discussion is ended. I don't grade individual posts (not possible in Canvas), so I just hold grades until the discussion is closed/end date. And then it can take a few days delay until I manually work through each student and get the grade posted.
If the setting were based on "score at least" that could delay student progress while I am in the process of grading.
Of course, a release based on grade could always be an instructor OPTION to use or not.