Junichi Masuda, the producer of Rhythm Hunter: HarmoKnight stated the below about releasing this eShop title outside of Japan:
“In terms of a North American or European release, it’s definitely something we’d like to do, but we’re working out the details with Nintendo now and seeing what would be the best way to get that to players.”
Looks like we might be seeing a release of Rhythm Hunter in North America and Europe sometime in the future.