Quoted By:
I think it's because vtubers are more of a mainstream thing in Japan, like anime.
So more normal people are into it, these are Nijisanji's fanbase in Japan.
Hololive's fanbase are more like otaku who are more deeply involved with anime, idols, etc.
In the west Vtubers are overall more niche so you have more people who are into stuff like Hololive instead. Additionally Hololive became massive in the west before Nijisanji was even known by most vtuber fans. This led to a lot of people becoming Hololive and hating anything they see as competition, such as Nijisanji.
As NijiEN has grown in numbers these people have become more aggressive tribalists.
Nijisanji is also breaking many of the unwritten rules of vtubers, this sort of path always leads to a lot of opposition.
They pioneered having male vtubers when the entire western industry was nearly 100% girls only. Then they normalized interactions between girls and guys on stream. They also pushed the envelope on various topics like being open about LGBTQ+ support, talk about sex and off collabs. Stuff that's too "real" for a lot of the fans that just want this to be escapism and for vtubers to be disconnected from the real world.
For a community that is essentially born from Hololive having this thing that's pretty much the antithesis to it suddenly normalizing all this taboo shit represents a threat.
I know indies exist and Vshoujo exists but for starters indies are very small so they don't represent a threat, and Vshoujo members are on twitch not youtube, they have a completely different community in there. And they aren't branches of Japanese companies.
There really aren't that many Vshoujo/twitch indie fans in /vt/.
People do hate them but without fans who oppose the haters the hate doesn't do much it's just background noise people ignore.
With Nijisanji there's an actual fanbase here to clash with the rest of the board.