GameCube titles are odd like that, but it is important to remember that even though these games are the best selling on their consoles that GC was still easily the lowest selling console of the generation, so compared to their PS2 and XB contemporaries these games are harder to find.
Actually, I believe that Xbox and Gamecube sold about the same.
gamecube sold about 30% less than xbox in the states and europe. in japan on the otherhand xbox is uber rare compared to gamecube.
In North America, it honestly feels like GameCube was a bit more obscure than that. Walk into any given game store and the GC selection is 1/3 the original Xbox selection, which itself is about 1/3 the PS2 selection. By that token, often a selection of GC titles are about 70% multiplat shovelware, whereas the availability of more premium titles on Xbox and PS2 is typically a lot higher. GameCube feels almost as obscure as Dreamcast. Maybe not that drastically hard to come by, but damn near it for a system that seemed to do okay back when. This is not a recent trend either, I never had much luck finding GC games, even back when I first started to collect.
Ironically, I found a stack of some pretty decent ones sitting out by the curb, waiting to be picked up by the garbage disposal this year, including Melee.