Can anyone explain why the wrong ones are wrong.

Incorrect: Julie was able to climb the tree as fast as her brothers.
Correct: Julie was able to climb the tree as fast as her brothers did.

Incorrect: The blue dress looks more flattering on you than the red one.
Correct: The blue dress looks more flattering on you than the red one does.

Incorrect: Three times more students attended prom this year than last year.
Correct: Three times more students attended prom this year than did last year.