what makes buttercream crust?

I know it's something to do with the sugar/fat ratio, but of course I'm not smart enough to know exactly what

Crusting occurs only in fat (shortening and/or butter) plus powdered sugar icing. Basically, there is not enough moisture in the frosting to keep the very outside of it from drying out.

thank you both

I've always wanted to know what makes it do that!