Hmmm, i'd think specific design would account for about half of the different shoes / shoe types on the market, and marketing and advertising would account for the other half of the differences. Maybe I'm being cynical, but there's so much marketing BS when it comes to sports shoes it's not funny.
Point in question (which is brand oriented, but still holds relevance): Nike makes maybe a couple of dozen running-oriented shoes a year, maybe more. Go to a marathon or triathlon and look at the front 80% of the field and do a count.... A million Asics, Brooks and New Balances, and the very odd Nike here and there. Why? Because most of the "specific design" nike put into their "running shoes" is designed to make it feel cool & squishy in the shop, and look good. When someone has 40 kilometres to run, they care more about support & strength in a shoe, than squishy energy-wasting crap that is often really bad for their feet & joints!
[Note: I haven't been to a marathon / tri in the US. I hope that the US population aren't swallowing the Nike BS just because Nike are Nike!
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