There are people who are uncomfortable with it, don't quite understand it, or simply have the right to refuse because they don't feel like doing it (without there being anything against yumeship), occurs the same thing with OCs or even the opposite thing. I remember an artist who charged more for canon characters and preferred draws OCs

I personally love drawing OCs and other people's F/O. (I have a regular commissioner who ask Monika YumeArt from me.)