@Awoo
It's the big perplexing thing about Hololive English... there's a lot of Japanese.
While it caters to the western, English market, it also inherently caters to weebs. And you can't cater to weebs without putting some Japanese in there.
Also I'm sure there are a lot of Japanese fans of HoloEN too. So all the more reason.
@Awoo
Also, the song is a Hatsune Miku cover...