Because The Sorcerer's Stone is the first film in a ginormous franchise, no one wanted to get fancy with the narrative. (They left the magic and tricks for the wizards.)
The narrative technique needed to deliver the facts as expediently as possible, so Columbus & Co. stuck to the old standby: third-person omniscient. That's the way Rowling wrote it, after all, and Columbus clearly saw no reason to fix what wasn't broken.
We largely follow Harry, moving away from him every now and again whenever there's some bit of information we need to know.