The real answer is simple. The Taiwan yab. That was basically the defining moment when the fanbase was united and the holobox was solidified. Being up front, outside of 5 talents (Pekora, Marine, Gura, Suisei, Korone) I can't really say that the other Holomems is better than the rest of the vtuber field. What really made the fans become diehards was the whole Taiwan debacle. You weren't just a passive viewer at that point, you actively wanted to support and see everyone through that ordeal. Add to the fact that you had major corpos like Asus and Mahoyo, not to mention other vtuber companies lessening if not outright cutting ties with Hololive itself (e.g Mea, Nijisanji sans a few livers, even Noripro for a time) and it really felt like it was Hololive against the world. And that kind of scenario really captured the imagination and Saviour fag tendencies of the fanbase. Its the difference between cheering for your favorite player regardless of what team they play for and cheering for your hometown team that you've known since forever. That's also the reason why I seriously doubt that any vtuber company will ever be able to replicate the holobox as a whole. VShoujo had the perfect opportunity to maybe tap into that with the whole Silvervale yab , but actually having to do it is a different story.
