Both third person and first person narrative modes have their restrictions. Third person presents events from a point of view outside the story; first person shows them from a single character's viewpoint, one who often has incomplete information. Changing third person to first person will create further limitations for a novel's viewpoint and voice. Some writers prefer those limitations, but some bridle at them.
Third to First Personalizes
First person voice is often more engaging than third person, and invites the reader to identify with the narrator. Compare Mark Twain's third person "Tom Sawyer" to his later, first person "Huckleberry Finn." The former is a formal presentation of a boy's adventures; it treats Tom's escapades satirically from an adult viewpoint, and little besides whitewashed fences and Indian caves is memorable. "Huck" is riveting, because we enjoy and identify with Huck as an outsider and appreciate his dialect, his rebellious spirit and his overtly honest outlook on the adult world. Tom is enjoyable; Huck is classic.
Mysteries, Third and First
Mystery stories gain different perspectives when told in third or first person. Third person narrations often embrace broader subject matter -- Dorothy Sayers' "Wimsey" stories add sociopolitical commentary to the mix, although they still function as mysteries. Changing a mystery to first person creates an ideally limited narrative: Conan Doyle's Sherlock Holmes stories fool the reader by having him identify with duller-witted Dr. Watson, while the author plots the sleight-of-hand that creates mystery. In one unorthodox book, Agatha Christie overturned a cardinal rule, making a first person narrator her murderer.
First Person Resists Scope
In "On Writing," Stephen King, a first-person-mode detractor, notes that changing third person to first person limits an author in terms of what he can "show and tell." First person is confined to one setting and one set of actions; for a tale of massive scope and panoramic events including numerous characters, such as King's "The Stand," third person is vital. On the other hand, one author who did create a panoramic novel in first person is William Faulkner in "As I Lay Dying," in which each chapter is narrated by a different character.
Omniscience Sacrificed
Omniscience is one other device that a writer sacrifices in changing third to first person, but this is more than fair when you consider that she can tune in on one person's thoughts instead of many. Harper Lee's "To Kill a Mockingbird" would be unthinkably confusing with the thoughts, not only of Scout, but also of Atticus, Jem and Boo; Ian McEwan's "Atonement" would not exist as a story without the penultimate mental confessions of its guilty narrator.