Could be that not all people can become animagi, don't have the time to do so if they can, or simply because it is an incredibly difficult skill to master. In regards to Peter, it could be that the other two mastered it first and coached him through the rest or that he has a talent for transfiguration (I'm not convinced that he's completely hopeless at magic considering he did become an animagus and showed proficiency with the Unforgivables - circular logic or what? ).