Now before I start answering this question, I want to state that I'm not gonna bash on Korea, their music, or their fans. It would be off topic to do so and unnecessary. I also want to state that I am a solid jrock fan and a mere listener of Korean music.
Anyway, the question on some people's mind is "what's going on?" and "why is everyone oogling these pretty boy dancers?". Well, I have asked around and let it settle in my mind the reasons. So, now I'm ready to give you my opinion on the matter.
Everyone's different, we should all understand this. Not everyone thinks that Dir en grey is the most superior band in Japan and not everyone wants to get in Heechul's pants. Therefore there is no one answer to this and since I don't delve into the goings on of the kpop world religiously like jrock, excluding one bias, I'm basing some things on pure observation and some input from friends.
I can honestly say for the past couple years or so, jrock/visual kei as a genre has been waning. From the economy, major bands breaking up or separating, and visually appealing j-indies bands with no talent overshadowing those who do. This could be the reason that elitist and jrock lovers are switching to kpop. Although, with a few iconic comebacks like Luna Sea, X Japan, and Seikima II, jrock could be making a slow gain to make up for the losses we've experienced since 2009.
Another possibility that somewhat ties into the first is boredom. Many good bands have been disbanding or building up the energy for later while the latter distract fickle, superficial fans and dominate the genre. This may turn off potential jrock fans because of the terrible representation that's flooding communities and forums.
There is also the increase of kpop invading Japan. Sure, a few groups and artists have dabbled back and forth between countries like DBSK, Big Bang, and BoA, but now groups like SNSD and KARA are taking over the charts and pushing some jrock bands on the back burner. Unfortunately, this issue has formed a conflict between both fandoms despite the matter of it being a tad hypercritical. This matter could also lead to another factor for the switch which is that the genre is a trend or that crossingover is.
If you've seen the movie Suicide Circle (a.k.a. "Suicide Club"), the main biases of the story, I believe, is that once one set of people establish something popular, the rest of society follows. It is human nature for us to be fickle and wanting to be accepted, so we change parts of ourselves to fit what is trending. Some more than others.
There is also more acceptance of fangirling in the kpop fandom. No one will criticize you for making shallow comments about someone. Maybe because a big part of the genre is based on image. I know I've seen some very pretty boys dancing around and singing out of tune and yet girls still go insane over them. Lastly, there is less personal boundaries. You can find out more personal information about your favorite idol on the internet and see what they're up to on social networks. During interviews, they're a little more open on what types of girls they like, embarrassing moments, and items they keep in their bag. But I think that's a cultural thing.
So, what other reasons are left aside from the crippling jrock genre and fandom, boredom, misrepresentation, the Korean invasion, trends, the freedom to fangirl, and more openness with your favorite idol's personal life? Well, there's my reason for why I enjoy it. I like trying new things because I love music and some of the Korean music I enjoy has opened my eyes and ears to other elements I enjoy about music as a whole. It also sometimes resembles parts or influences of things I already like from other musicians. There's a new, refreshing, and different structure to kpop compared to jrock. I also get to learn what is appealing to a different culture. I'm sure some kpop fans feel the same way when discovering jrock.
No matter what the reasons may be for switching over to either side, I think we need to stop bashing each other for enjoying something completely opposite from your main musical taste. We should also stop with the verbal war of kpop staying out of Japan, because the statement is rather hypocritical and music is a great way to unite the world. Lastly, it's okay to enjoy both as well. I'm pretty sure there's no law against liking opposing genres. Be open-minded~