If an illustrator is working traditionally with watercolors, oil, acrylic or some other medium, it will be difficult and frustrating if the writer wants a full moon instead of hangnail moon - especially if there is no room in the artwork for a full moon. The piece might have to be started over. This will make both parties unhappy and a bit frustrated. Whereas a digital artist could fix the problem in 5 minutes, apply a color shift over the entire artwork, move text around, change fonts etc... the possibilities are endless for digital artists.
Most computer art programs have texture brushes that simulate traditional brush marks beautifully. So stop your tomfoolery, find a good artist - and make sure the artist can work digitally.