As much as we're shitting on Kronii for this yab, I would say she only takes on 20% of the blame here.
A lot of people forget the original reason for male collabs on the JP side. There were very few VTubers in existence during the period of time that the 0th to 2nd Generations were operating (2018 to early 2019). Niji accelerate was still in the early phases and JP only had around 13 talents (Suisei, Miko and Azki were in different projects during this period).
And so at that point, they only had each other as potential collab partners, and since most of them streamed every day (big breaks could kill your channel) they eventually ran out of collab ideas and partners and had to look elsewhere. They tried with Holostars at first, but that only got the stars flamed from every direction, so they resorted to collabing with others outside of the company, males included - which notably had way less flame for some reason.
But once Gamers and Sankisei debuted in 2019 and Miko + Suisei joined, the roster reached 23. And then they had more than enough people within the Box that they could have a lot more potential collabs without ever running out of content.
In addition to this, once the Holocaust occured, Cover started to do everything with a paper trail. And so they had to vet outside collaborations a lot more, introducing a bunch of bureaucracy in front of outside collab seekers which also served to diminish the interest.
The CN yab also accelerated this circling of wagons. When Hololive was blacklisted for a period of time by most other JP companies, they only had themselves to collab with. But by that point, they had more than enough people that they would never run out of fresh collabs and collab groups.
Subaru talked about this bureaucracy issue as the reason why she reduced her collabs outside of Hololive (among other reasons). Botan talked about how she couldn't control how people outside of her Box would behave during collabs, and that it was a lot "safer" and "comfier" to collab within the box, because any issues could be resolved internally. Tamaki from NoriPro also talked about why he wasn't collabing with the Holo girls as much, because they were freaking busy as all hell (this was around the time of Holofes preparations though).
So in JP - more than the idol angle - I would argue that these were the main reasons that male collabs (and collabs outside of Hololive in general) died off -
Bureaucracy :
https://www.youtube.com/watch?v=vIvuGHKgw0wComfiness/Lack of drama :
https://www.youtube.com/watch?v=-bIQIG-uYG4Being busy with stuff :
https://youtu.be/sX9kuXL0ZhM