No one thing caused the shut-down. As best we can determine it was a combination of Cobra's ill health, chronic funding issues, incessant hacking, and other stuff.
Also I dispute the OP's assessment that WWOEC had been irrelevant "for years". We had a vibrant community with discussion boards on every subject, multiple updates every day , and a large productive stable of artists that produced many fine works. I don't see how you can consider that "irrelevant".