It might have been different if there was a demand for 3rd party games in general as well.
But how many 3rd party games sold over a million copies on the Gamecube?
How many non-shovelware 3rd party games were released within the first 12 months after Cube's launch? "Fool me once, shame on you. Fool me twice, shame on me." Within the first year after launch, the Gamecube 3rd-party selection was like a "Who's Who" of the worst cross-platform games. After seeing what the Gamecube was capable of with Rogue Leader, a craptacular port of Spyhunter isn't going to cut it. After a year of generally horrible cross-platform software with bad graphics and broken controls, cross-platform games gained a reputation of being junk.
If you frequented Nintendo boards back when the RE4 port was announced, a lot of Nintendo fans decided not to buy the game
on the assumption that the graphics engine would be dumbed down for easier portability to the PS2, and that the PS2 would get far more content. Thankfully, they were wrong about the former (but would have been right if RE4 hadn't basically been completed), but they were right about the latter, and this before the announcement of Assignment: Ada! "Cross-platform" on Gamecube was synonymous in most people's minds with "Crappy 4-bit textures, no water effects, unstable framerates, mediocre lighting, broken controls, and reduced content." You can't blame the fans for that. Blame the total crap that developers were putting on shelves.
Developers need a reason to give a Nintendo console a chance, and Nintendo fans haven't given them the most important reason of all.
Because developers generally haven't given them a reason to buy their games. To get a spoiled Nintendo fan to buy your game, you have to put in some extra effort to make it
not suck and
have decent graphics. This is far, far beyond the ken of most developers. Certain 3rd-party games sold quite well on the Cube, and no one seems to understand that it's because they didn't suck and got decent publicity.
Publishers and analysts think that games should sell simply because they're on the shelf.