Besides selling better than the GameCube, it also was just taken much more seriously as a PlayStation competitor in the public discourse. Especially because of Halo, which was undeniably one of the biggest new IPs introduced in the 6th generation.
Another point was that it was only Microsofts primary system for 4 years, while the Gamecube was Nintendo's primary system for 5. The Xbox still had some momentum when it's successor released, while the Gamecube was completely dead.
All that being said, I am not really sure I'd consider the Xbox a successful console though, it wasn't a failure either, just somewhere in the middle. It was a strong stepping stone towards the 360, but I don't think the sales justify calling it a success, considering what Microsoft's goal was. For other companies, 24 million systems would be a huge success, for MS, not really.







