The Game Cube was more powerful than the PS2. The Xbox had a hard drive, and online network, that made it in some ways more powerful than the GC. Overall the GC could push higher poly limits than any last gen system. The Wii and 360 are using software used on their previous systems. Developers will feel more at ease making games on these systems.
Over 1 year ago people thought the Wii would be dead last on the market. A lot of companies abandoned the idea of even making games on the system. However, those same companies, THQ, Activision, EA are turning around, and made entire studios dedicated to Wii games.