I think management has a few girls that simply don't get promoted heavily in singles despite being rather popular with the wota. For me it has always been very surprising that Maria, Sou Janai aside, got very little attention in singles. Same happened with Duu who only started becoming some middle-tier member after Riho and maybe even after Kanon graduated.
These girls do well in other aspects of idol life, for Maria it's photobooks, for Chii handshakes, for Duu it was acting but probably goods in general too.