It's not that surprising if you ask yourself the question of "What does Hololive actually provide for their male talents?"
Pretty much just the blue dorito exposure buff, but even that is massively diminished compared to the numbers a female talent expects. Only the absolute best can rise above the male debuff to reach numbers a female Holo could take for granted.
In exchange you have extensive restrictions on your ability to stream and create content even on a basic level. What content you make and how, what's acceptable (which can be a good thing, but can also be pointless) what games you can play has to be run by permissions managers who by now are infamously incompetent, opaque, and selective in their attentions. Moreover it's for regulations and culture that don't even apply to 99% of non-JP talents. Regular make-work meetings, typically on JP time, and work on "projects" that more often than not seem to go nowhere for the same reasons perms do, byzantine middle-management decisions.
The other biggest draw for a Holo is the built-in community of other successful streamers to cross-promote off and do events with, but this is denied to the males. A significant part of the audience is explicitly hostile to your mere existence, and the rest are expected to politely ignore you in most cases to the extent that acknowledging your "coworkers" or being acknowledged by them can be a statement and minor controversy in itself.
Lando was in the interesting position of having his RL persona be successful in itself, so he had something to compare the benefits to. And I don't think he was too off the mark when he called Magni a side job, one with a lot of expectations and a shitty uniform. Vesper gained in exposure but he ran into the worst of Holo management, so probably followed Lando's lead here or they otherwise fed off each other, and Randon has a lot of eyes on him right now and could retain those numbers if he plays his cards right. Male vtubers aren't that rare or unsuccessful in the indie scene, hell I'd say even Niji does them better than Holo on the whole, so why not make the move and no longer have to deal with the bs?