Differences between Japanese curry and American/European ones

I regularly eat Japanese curry, and sometimes Indian curry. Though I cannot explain well difference between them, I know it. And, I don't know well American/European styled curry.

I'm surprised the community people likes Japanese curry much more than I expected. As I thought there are little differences between Japanese and American/European, I've never expected Japanese curry pics gain a lot of upvotes. Just due to katsu or korokke toppings?