And no, making more money out of this final chapter of the Harry Potter saga is said to have nothing to do with it. The problem is something that most fans of the books have noticed in the film series: that with each new chapter in the movie version of the story, whole chunks of pertinent details from the books are getting cut out. This was especially a problem with this past summer's movie of Harry Potter and the Order of the Phoenix. Not to mention the fact that Deathly Hallows is an incredibly dense book that ties up every thread that's been left dangling across the previous six books. There's no way that all of that could be wrapped up in a single two-plus hour movie. So doing Deathly Hallows as two installments makes some sense.
I think that someday, there's going to be another film adaptation of the entire Harry Potter saga. It'll be done with the entire scope of the books in mind, now that they have all been published, instead of what has really amounted to a lot of guesswork by the various directors with input by J.K. Rowling... who naturally couldn't lay all her cards down on the table for the films' sake. But ten or twenty years from now... I think the time will be ripe then for a cinematic treatment that will be able to do the Harry Potter saga its fullest justice. That's not to slam the current batch of movies, 'cuz I absolutely do love them quite a bit. It's just that later on there'll be the proper hindsight to handle them better. And maybe by then, the groundwork will have been prepared so that Deathly Hallows could still be a single movie.