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No one has said the actual reason--HoloJP has reached market saturation and management is looking for more avenues for growth, so they're probably encouraging certain talents (especially the ones who already have done Stars collabs) to venture outside the holobox.
On the EN side, both unicorns and homobeggars have also adopted a somewhat one-dimensional view of "idol culture" that means no male interaction ever, when in fact things like being interviewed by a man or appearing on a game show with men or co-hosting an event with a man have always been normal for idols; the stuff that's seen as unacceptable is dating and casual fraternization. In a Hololive context, taking Koyori as an example, she's co-MCd a Power Pros tournament with a man and interviewed the designer of her branded sake (a man) recently, but avoids casual "play" collabs with Holostars or other male vtubers, which her fans almost uniformly accept, but ENfags made a dozen bait threads about because they don't get the nuance.