As for why they’re popular in Taiwan…I’ll take a random guess here.
I would figure it came about as barefoot running, more minimal running shoes, or at least shoes with smaller heel-toe offsets became more popular. Some people probably figured if you can run barefoot…why not just run in flip-flops and made it work, after all most people here walk around in flip-flops anyways. Especially since running around barefoot in the roads in Taiwan there’s probably just too much junk and dirty stuff no matter how much you look and keep your eyes out for stuff.
So I would figure as a few people started doing half/full marathons in them they became popular/trendy and other people started trying it too. As more people tried it some companies probably made some of their own that are a bit more suited to running or less likely to flop around or rub so hard against the skin. Also a lot cheaper upfront than running shoes.
I somewhat guess it’s similar reason (price and easy availability) that rain boots are so popular for hiking here…never seen that anywhere else from what I can remember.