Because they can't regurgitate existing fanbase if they want to continue making money. Nijisanji's strategy was always attracting new people, even if in small amounts, with debuts. And who this new people are going to pick and simp for, new shiny cool vtubers from a big respectable company who only releases someone once a year, or another dime a dozen from Niji who shits them out every other month? All fanartists are drawing them, all streamers will be watching them and talking about them, all of your friends will also be all over them. It doesn't matter they are EN either, japs have no problem watching someone they don't understand, as evident with both Holo and NijiEN