Apparently he originally used to write in English first and then translate into Japanese. His English isn't good, so it forced him to write in a highly simplified fashion. It seems he now writes directly in Japanese, after he settled on his style. I haven't read more than a page or two of his books (browsing the bookstore) and I found it extremely easy to read (in Japanese). I suspect that's the appeal. Like you, I'm not a huge fan, but the number of novels I've read in Japanese is fairly small in number so I'm not the best judge of Japanese literature :-)
For text in Japanese I only read a few pages in class and during exam and that wasn't relevant to my interest either: the style is flat and he use to many loan words from English. Up to that point I wonder where his success come from because I really don't get the appeal of his work (from what I read).