my bad, I keep on mixing up Yukari & Youmu names because Yukari last name is Yakumo xD Youmo & Yakumo names are too close to each other dammit, all these Y names xD
why do you prefer Yuyuko x Youmu over Yuyuko x Yukari???
Part of it is the difference between them. While Yukari and Yuyuko have a lot in common, and in most doujins a history as well, I think they're too similar to be that interesting. They've both got similar looks, similar positions as heads of households/realms, similar power levels, and even similar flighty/naughty personalities (although Yuyuko wins on the eating front like woah).
With her and Youmu, there are more differences to play off of. Master vs servant, dead vs half-dead, bold vs shy, experinced vs naive, etc. They've also visually much different, which works well in the art. As well, since their interactions take place in a more intimate setting (living together, caring for the realm of the dead, etc.) there's a lot of opportunity for situations to crop up. Plus more chance for teh funneh of course.