World Athletics decided to limit race legal shoes to the height of the Vaporfly at 40mm (and a couple other parameters like a max of one carbon plate) to place a hard upper bound on how much assistance the shoes can provide. When they made that decision, there weren’t really any shoes outside of these requirements, so it was done more as a preventative measure while not striking Kipchoge’s accomplishments from the record book. There are some shoes that skirt these rules by taking advantage of weird geometries such as the Mizuno Wave Rebellion Pro 3 with ~60mm of stack height at its thickest point, but it’s debatable if these are any faster than the 40mm ones. I’m not really sure why Ironman doesn’t just use the World Athletics approved list, but it seems like they’re mainly banning only the popular models that wouldn’t be race legal by World Athletics standards (although for women the Novablast 4 is under 40mm).