to be honest... i'm not super versed in vocaloid .. "lore" for lack of a better understanding. so if there's a connection regarding their history that Could Possibly apply i'm not aware
however, i can come up with some explanations for why i think the vocaloids were likely chosen
1. miku. ichika. starts them off. i feel like this one shouldn't really be controversial of an explanation
2. rin and len. kagamines grouped together isn't exactly a hard concept to understand. i do think they're sortof generically troped into the "cheerful ones" which fits the upbeat nature of this group. also i think they wanted to generally avoid having groups with 1 male in it lmao. particularly for vocal reasons, again
3. luka. this one i think is aesthetic/vocal appeal purely. luka/honami/shizuku/mafuyu already appear in april fools under meryuu, it's the same specific appeal there if you understand what i'm talking about
4. kaito. i do think grouping him with men (rui/toya) is another pretty easy explanation honestly.
5. meiko. this one i'm... not totally sure on. i do think a lot of the time these are basically just vocal qualities mostly. she's a little.. "sharper"? i guess? which fits that group well
...so yeah. it's honestly largely just aesthetic/vocal appeal but the generic traits assigned/associated with some of them kinda match these characters.