Nintendo was stuck in the red ocean in the console wars during that time (remember this was pre-Wii where they hadn't reinvented themselves yet in the home console space and had yet to really separate themselves from Sony and MS outside of having pressure-controlled triggers and a lunch box console with a handle). Whereas in the handheld space they were essentially alone in the market, and also had a larger library with more and better games overall. The GameCube felt more niche as a whole - it had the stigma of being a more "hardcore" as well as kiddie console, while Sony was more mass market with their PS2, and MS had the "cutting edge" tech and superior online angle to tout, which at least drew a little more attention.
"We hold these truths to be self-evident - all men and women created by the, go-you know.. you know the thing!" - Joe Biden
