BTS is a K-Pop (Korean Pop) boy band ala NSYNC or the Backstreet Boys. Even though they mainly sing in Korean they reached #1 in the album charts here in the US. I.e. they have a huge US and international following and are in fact more popular outside Korea than in.
Reaction videos are...reaction videos. Don't really understand their appeal myself but I'm an old anti-social fart.
As for reaction videos to BTS showing some skin just imagine if YouTube was around in 2000 and NSYNC uploaded their music videos there and, say, Justin Timberlake, flashed some skin in those videos. That would get the exact same reaction.