I guess the PS2 was just too strange a machine to emulate easily, while the gamecube used a pretty standard CPU ISA and a basic off-the-shelf GPU. The PS2 has a relatively common CPU ISA except for all the vector instructions added by Sony then a relative ton of exotic components.
It probably also helped that the Gamecube was cracked early on, leading hackers to get Linux running on it relatively quickly. There was PS2 Linux, officially from Sony, but that didn't really help you get to stuff without drivers. I guess also the Gamecube's CPU was more powerful than the PS2s one, so if all you have is that it's a more attractive environment to hack around in.
Then the fact the same architecture, more or less was recycled for the Wii and its successor, so the community was able to move on wholesale.