I am Japanese and and started wearing Speedo-like suits as a boy when I was on swim teams (form age 10). I was a diver and we always wore speedo-type suits for diving (and they are still popular with divers) and the main reason is that they just give your body a real streamlined look as you enter the water which is pretty important to score well in a dive.
Even more, if you are doing dives from high, like a 10 meter platform, if you wore jammers, they would get stripped right off you when entering the water
!
This is me at 15 (2nd from right) with the goofy haircut, and my little brother to the right of me, and other team divers, and the love/hate 10 meter platform behind us.
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And this is me a couple of years later...and yes, embarrassingly with a slight boner LOL. Yellow kyoupan hide nothing! That seemed to happen whenever I was scared of a dive from 10 meters! Go figure
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Speedo brand wasn't really that big in Japan. If you look at my team picture you can see we mainly wore Mizuno, Arena, and Asics. Suits by these companies were always way more skimpy for the Japanese market than the ones they would sell in America...and they still are!
In Japan, kyoupan are still worn by many guys, young and old alike. There's no prudish stigma about them the way there is in America, and they are just as popular today as the used to be really.