From my point of view as a person who actually rarely watch his streams, for me it's because he streams like almost everyday (& even everytime) that made me kind of lost my interest to follow him in his every streams (note that I still like him, I'm a Holostars DD).
You might think "but Roberu's frequency of streaming is/was the same as Aruran, yet theres a lot amount of people that still watch him than Aruran"s". Aruran's streams are chill, but that could also be his disadvantage. It seems like lots of people would prefer chaotic/random streams like Astel & Roberu's, hence why people usually watch their streams (if I recall the last Among Us collab stream, they both got the most viewers, over 1k)
Another reason is, every Stars have their own unique "outlier" loyal viewers. By outlier loyal viewers I mean theyre not "in" Holostars fandom, but still manage to keep up some of the Star's streams. Izuru & Rikka has lots of those viewers for their singing stream. Roberu, Oga, Shien, & Astel's also have lots of those viewers, the channel names that maybe you couldnt see on any Star's streams except in themselves streams. Meanwhile Yurustars, the viewers are mostly within Holostars' fandom, maybe not even their oshis, that come frequently to their streams because their oshis not streaming.
Tl;dr: even if he is hardworking and has nice streaming atmosphere, if he can't attracts people outside of his "zone", then it most likely to make his growth slower than the ones capable.