Natural selection. A ribbon-less Reimu would have to be stronger and smarter to survive. That would also include extremely aggressive behavior to keep the punishment happy yukkuris away. That 5% would have no choice but to become monsters.
Not a coincidence - I was trying to figure out what made his yukkuri so painfully adorable, and I think I made some progress in figuring it out. The double-spherical colour gradient radiating from their cheeks give them the appearance of being nice and plump. Then of course, there's the fact that he can actually draw, unlike me.
When it comes Kiri's colour drawings , then yes , that effect has a lot of impact. But remember that his style also works in traditional black and white artwork , like this post #27939 or this here post #23303 Shading seems important too , but I think it mainly come down to the overall shape of the yukkuris and the eyes , especially with the ko's. The he draws them in their 'perfect roundness' and how their shape distorts according to the situation to emphasize their squishyness. Ridiculously cute expressions and puppy dog eyes also help.