Adult Protagonists in Picture Books

Just because PPBF is taking a summer break I certainly haven’t stopped pouring over great picks. This week I came  across this beautiful award-winning book written by Lore Segal and illustrated by Paul O. Zelinsky (published in 1985, reissued in 2005) and had to wonder, once again, why is it that editors shun adult protagonists? Are children really unable to relate to them? Why are some of my favorites still so popular? Any one have a title they’d like to add?