Why aren't thong style panties more popular in Japan? Why are pantie-lines ok in such a fashion forward society?

This is a totally weird question, I know (and I swear I’m not a pervert), but I’ve been an exchange student in Japan for almost a year now. I really love Japanese fashion, so I pay attention to what other girls wear and have noticed a lot of really unsightly pantie-lines on otherwise pristinely dressed-up girls. Since these girls put so much effort into making sure their hair, make-up, and other parts of their appearance are flawless, it doesn’t make sense that they would fail to notice the lines of demarcation on their rears.

I’m starting to wonder if it’s attractive to be able to see the shape of a girls underwear, or if more discrete underwear just isn’t so popular in Japan yet.

I’m not sure if it’s possible for anyone to really know anything about the subject, but I’m interested in hearing some theories! Thanks!