I think they need to have something to measure each applicant across, and then offer a variety for applicants to choose which one will best show their story. I think it has more to do with strategy than with weighting - if they gave you choices for the first 2 essays, then there would be no benchmark to measure students. A mistake and 3 accomplishments are 2 generic topics that everyone could answer, with the other 2 essays expanding on that and adding perspective to the story.
Sure, these first two questions can very well change in the coming years, though I doubt they will ever remove the constraint that everyone has to answer them. These 2 essays put everyone on the same playing field.