Maybe because of its light and fruity taste, which suits the Japanese palate. And also because it is trendy, something that excites their palates even more.
The thing that puzzles me, however, is the cost of the new Japanese nouveau wines. I thought the French Nouveaus were expensive in Japan because of the priority air delivery. Which is why so many nice gifts were being attached to them (designer drinking glasses at 7-11 etc.)
I saw a news story about the launch of Japanese Nouveau wine, so was curious about it and checked it out. Even more expensive than the imported French wine!
I suppose Nouveau wine in Japan is not only so popular because of its taste, but the trend comes from the days when only snobs and the elite could afford it. I don't know how long you have lived there, but when I first came, over 20 years ago, it was very hard to find wine and if you did, it was ridiculously expensive. Now you can get good quality cheap wine everywhere. Bet you that if Beaujolais wines were cheaper than now, they would drop in popularity ... there is some kind of mystique attached to them and lets everybody be a snob in November.
I am a wine lover and I quite like the Nouveaus(x), but I do prefer a good red Bordeaux or Chianti which I can get for half the price.
(P.S. Sorry to go on, but I did think at one time that the young, fresh wines had fewer chemicals in them to preserve them. Or even none at all, as they are meant to be drunk quite quickly and so wouldn't need preservatives etc. That got me excited about them, but I was wrong. Thet still have those nasty sulfates and things.)