I have a 5 year old daughter and an 8 month old son. We read at least 20 minutes a day with them and sometimes more. That adds up to a lot of books over 5 years, especially when you consider that some of these books only take 5-10 minutes to read.
The truth is most children's books are "bad" from an adult perspective (not that the kids care). The art is amateurish, bland, or uninspired. The stories are cliche and boring. I know it sounds silly to say such things about children's literature...
Until you pick up some of the best work by Dr. Seuss. It's like comparing Pixar to Saturday morning cartoons. Seuss' illustration is original and engaging. The Wordplay is fantastic. The stories' morals are layered and complex (Every entrepreneur and consumer on this board should read The Star Belly Sneetches immediately, if it's been more than a handful of years) which is crazy considering the simplicity of it all.
It's not all gold, Dr. Seuss' body of work is massive and naturally there are hits and misses. But the hits are ingenious and work on multiple levels for multiple audiences. And the misses aren't bad. It's like your straight-A student bringing home a B.
He's not the only game in town, of course, for timeless and truly ageless (ages 1-death) literature. Shel Silverstein gives him a real run for his money (and has a wicked subversive streak that Seuss is lacking, if that's your thing).
Anyway, as a parent the durability of Seuss' body of work is easy to understand with the sociology and marketing analysis hijinks. The statistical dominance is quite impressive, and is surely a result of these multipliers acting on a product of unmatched quality.