Here's why:
* The Decepticons: Most of the new characters were Decepticons, like Shockwave, Sentinel Prime, Soundwave, and Laserbeak. These characters were already established in the Transformers lore, just not featured in the previous movies.
* Autobots: The only new Autobot introduced in *Dark of the Moon* was *Dino*, who was a small, fast scout. He wasn't really a major player in the film.
So, while there were a lot of *new faces* in the movie, they were all pre-existing Transformers characters.