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Hololive has more socially retarded streamers, while Niji streamers have a lot more normalfags (with the exception of a few really good ones like Tsukino Mito).
The western audience that would watch Japanese vtubers are not into normalfag shit, even if it's Japanese. Hololive girls know not to bring normalfag shit into streams and therefore are closer to a 2D fantasy woman.
There are exceptions of course. Personally, I think Towa is a better fit as a Niji streamer. She has that niconama background and closer to a normalfag.