Salte already said the reasons why, they're quiet, don't stream super-often, and/or have specific niches. Neither of them have larger-than-life personalities like your Pekoras or Marines, which means people can't draw "memeable moments" from them to expand their viewership via word-of-mouth, and they kinda just stick to what they know. Pretty much all Choco does is Ark, Minecraft (and she pretty much just started that back up), Apex, and ASMR. Mel has a lot of other responsibilities and content to create, not to mention traveling into Tokyo for work (which is alleviated for now since she rented an apartment in the city until the concert is done), so she hasn't been around much.
As for dedicated channels, a fella with a channel name by the name of "ルイス" popped up on YT a couple months ago doing pretty much dedicated Choco translations, with a couple other clips smattered around in there too. So check him out for some sensei translations. Still wish there was more of her though, since she's a really interesting person and commands a great deal of respect not just from the other Holos, but from the entire JP Vtuber scene. Sensei is multifaceted, hyper-intelligent, talented, and just a good woman.