The Mirror of Erised showed Draco Malfoy's father. Not just his father, but him looking upon his son proudly, telling him he's glad to have him as his son and that he's proud of all he's become. The boggart showed him his father as he was. Disappointed, snapping at his never-good-enough son, never as smart as Hermione Granger, never as daring as Harry Potter, never as loyal as Ronald Weasley. And the most devastating thing was, his worst fear was something he was used to.