I did not know this, but according to the storyline from the original Iron Chef, the show ended because Kaga died from fugu poisoning. I looked around, and that's reflected here, here, and here. It's also in the Japanese Wiki article for Iron Chef ("主宰である鹿賀がフグの毒に当たって亡くなったため、その甥が後を引き継いだ")
Freaking crazy, man. I know Kaga didn't actually die, but I had no idea they wrote him off like that. Did you?