It is the result of a flawed premise. Progressives believe that a "democratic" state means that public education, itself, must also be democratic and that connotes equality.
If education must be equal, then the educable must not show better results than the ineducable. Thus, the constant relaxing of standards until, at last, the intelligent are no better than the rest.
-— If education must be equal, then the educable must not show better results than the ineducable -—
That’s not really true. There are several nefarious currents running through the history of American public schooling. The central purpose has been to turn out indifferent ignoramuses as fodder for industry or government. The mechanism has been behavioral psychology.
Google John Taylor Gatto on YouTube, or read his book on line for free: The Underground History of American Education.