They allowed customization and the users went a little bit nuts adding glitter and flare and bad copy-paste code to their pages. So it created a kind of inconsistent experience where one page would be fine, then the next one would blast out some pop song and have a bunch of flashing gifs and broken scroll bars and shit. It was starting to turn into a bit of a mess and then Facebook came out with a uniform page design that gave people a more consistent user experience. This brought a lot more professionals and normies onto the site and that started to make Facebook more like an interactive address book with all your contacts, not just the kinds of people who would have a MySpace.