User blog comment:The Ultimate Dude/The Disappearance of Hatsune Miku - Again/@comment-4104705-20160214212125

Well, companies like EXIT TUNES haven't agreed yet to YouTube's new terms following the release of YouTube Red, so that's why they're being blocked in the US. The Japanese music industry has traditionally shown a stricter sense towards copyright enforcement and a resistance to most digital distribution of music.

Another reason could be just copyright strikes from the original creators.