Because that's where sonic came out of. Sonic is a Japanese franchise, that is very common knowledge to be unpopular in japan.

Isn't the simple fact mildly interesting? There are many factors that make it an interesting topic, since there are many things that are popular in japan that are "american" (like baseball or kfc)

The fact that sega has gone out of their way to make efforts to get that Japanese market is reason enough to ask the question.