A lot of reasons.
1) Calli has bigger dreams than hololive and everybody knows it.
2) Gura is a very supportive and caring friend to Mori.
3) Mori is, arguably, the only one that actually wants the exposure. Gura seems to resent her own popularity and the idiots it brings along with it, instead preferring her core of dedicated fans. Ame and Ina seem pretty comfortable where they're at. Kiara is fine doing whatever because she's just like that.