Simple answer is: the producers just choose good chefs for the show and give them the title.
There are only a select few "Iron Chefs". These are the mainstay chefs that continuously appear on the series. Any challengers who participate on the show cannot become Iron Chefs, but instead challenge them.
Iron Chefs from the original series include:
Iron Chef Chinese Chen Kenichi
Iron Chef French (I) Yutaka Ishinabe
Iron Chef French (II) Hiroyuki Sakai
Iron Chef Italian Masahiko Kobe
Iron Chef Japanese (I) Rokusaburo Michiba
Iron Chef Japanese (II) Koumei Nakamura
Iron Chef Japanese (III) Masaharu Morimoto
Iron Chefs from the series "Iron Chef America":
Mario Batali, an Italian cuisine specialist
Cat Cora, a Greek and Aegean cuisine specialist, also the first female Iron Chef
Bobby Flay, a Southwestern cuisine specialist and former Iron Chef challenger
Masaharu Morimoto, a neo-Japanese cuisine chef, and the third Iron Chef Japanese from the original series
https://en.wikipedia.org/wiki/Iron_Chef
https://en.wikipedia.org/wiki/Iron_Chef_America
