It's simple, they don't. There seem to be 3 different kinds of male Niji EN fans.
>Goons and Dachi
They left with their oshi, never had any love for Niji as a corpo, they just enjoyed that girl group. Now they branch out into other small corpos or other ex-corpos when they previously watched da wose for example on the side.
>other fans of the first 3 waves
Joined when it was all girls and cling to that past desperately.
>Scarle's fans
She stays exclusively for herself so it's like watching some female indie without all of that corpo garbage.
>What about the other girls that joined after Ethyria?
None of them have any following whatsoever, perhaps they retained some of their audience of pre-Niji days, but that's all. It's pretty likely that their viewers are just other Niji EN fans (female) that watch them on the side. Girls like Mogu and Sunny seem to do way better nowadays, now that they left this hellhole.