Male neckbeards buy TCG cards
Female neckbeards don't buy TCG cards
Nijisanji audience is like 60% younger female and will scale worse for specific streamers like Kuzuha, Kanae, Lun Lun.
Hololive can afford to do extreme neckbeard hobby like TCG, tabletops, DND and so on because their fan base are predominantly older males. And even Hololive were not sure (Lmao) that TCG is gonna do well until it got released
Sure there is a huge overlap/Niji still have holo-adjacent streamers with higher neckbeards rate like Hoshikawa, Shina Yuika, Ars Almal, etc. I guess it would be extremely awkward if you release an "Nijisanji TCG set" that is just consist of 10 female streamers out of 159 Nijisanji members. Doing the reverse and have single streamer collabs like this is the better move. Riverbear stopped baiting and making good observations today.
It's not about whether she is a fan of it, but whether her fans will buy it, like you said, Riku knows his audience.