According to the source:
Chris was unhappy with the financial arrangement within the group - he wrote all the music, yet the other three bandmates took an equal share in the multimillion-dollar publishing rights,
Audioslave formed in October of 2000, shortly after the break up of Rage Against The Machine. The group eventually recruited ex-Soundgarden vocalist Chris Cornell to front the band and went on to release three full lengths, which is exactly how many original studio full lengths Rage released as well.
The departure has paved the way for Rage Against The Machine's upcoming reunion shows, as well as guitarist Tom Morello's side project, The Nightwatchman.