It's fairly easy to see what's happened here. 'Legally Blonde' is right on the borderline of being a generic comedy, full of basic 'blonde' jokes and shallow characterization. In the wrong hands, it would have been. Fortunately for everyone involved in the production of the film, Reese Witherspoon was very much the right pair of hands, and she elevated the script. It's now one of the highest-grossing female-led comedy films of all time, and went on to become a franchise.
Witherspoon became a household name because of the film's success, which probably left Christina Applegate scratching her head. She'd been the first person to get the call from the producers, but dismissed it because she didn't want to play 'a dumb blonde.' Perhaps she didn't read the script all the way to the end. At least she's since had the honesty to refer to her choice as 'a stupid move' in the years since!