The claimed reason for the change, aside from the fact the FD apparently sucked, was that she wanted to go through Bella's senior year. We're not sure why she wanted this, since Bella's senior year was even whinier and more irritating than her junior year (if that's possible) but perhaps Meyer just didn't want to miss the chance to introduce irrelevant side characters which she could then write more truly awful books about. Apparently this makes up for the fact that making four books instead of two killed thousands of innocent trees who had never done her any harm, but it's cool; we didn't need that oxygen anyway.
It has been suggested by the sane that Forever Dawn should have been published, with a small modification to include the deaths of all the main characters. While this would have been a vast improvement at any point in the books, the suggested course of action has clear benefits over any other option aside from killing her in Twilight, since it would have spared us the intermediate books.