the established twitch streamers already have a good thing going, no reason to switch it up.
the ex-corpo vtubers just want back end support so they can keep the train rolling.
with ex-niji they have weird ass contracts that could possibly be preventing them from going to vshojo too quickly (the 6 month clause in the contract) so that could be why we see cases like mika/michi joining now, but pomu/mint being indie
with established indies like filian, onigiri, numi, shylily, etc., theres no big advantage to joining a group like vshojo. they already have a good system in place and likely have management or other 'employees' that they have to consider as well.
this is why the 'poaching' of ex-corpos seems to be the play for vshojo. they get a talent with an established fanbase who will inevitably do well, but dont have to worry about taking over the established infrastructure holding up those streamers