Picture books are often rejected for being "overly didactic." This is a word that gets thrown around a lot by agent-and-editor types, and I know if you aren't familiar, it might sound sort of harsh and insulting (particularly since, as I said, it is normally coupled with a rejection) -- so just what does it mean when an editor or an agent says Didactic?
It isn't meant as an insult. Didactic just means "intending to teach a lesson." (The word is also kin with "moralistic" - which is of course "intending to teach a MORAL lesson.") These words can have a negative connotation (like if I were to say "my neighbor came over and hollered at me for playing rock music instead of hymns - what a moralistic jerk")... but neither of them is inherently a negative word.
They are just negative words to an agent or editor because they are so very rarely are used in the same breath as "HOT!" "HILARIOUS!" "WOW!" or "AWESOME!"
Many stories are meant to be didactic or moralistic. Fables are meant to teach a lesson (and in fact that lesson or moral is very clearly spelled out at the end!) Some nursery stories, rhymes and famous old tales are meant to teach a specific lesson. Some sorts of modern "issue books" (like HANDS ARE NOT FOR HITTING or HEATHER HAS TWO MOMMIES or DOG HEAVEN or MIND YOUR MANNERS, BB WOLF or even EVERYBODY POOPS) are meant to leave the audience with a specific message at the end. Some of these books are so successful at their job that they are at this point classics and will be found in nearly every bookstore or library, and stay in print for ages.
These stories definitely have their place.
That "place" is usually one shelf in the bookstore or library. Probably labeled "special issues."
While the demand for them might be consistent, it is also comparatively rather small, which means they can be tough to sell unless they are REALLY well done, and the advances may be on the low side. The exception might be for a book by a well-established author or illustrator, or something featuring an already popular character, or on a "hot topic" possibly that can break it out of niche-ville... but I digress. The point is, because the market for them is limited, I believe very few (if any) agents would ever say that they are ACTIVELY SEEKING didactic picture book texts.
So if agents aren't looking for "teachy" -- what ARE they looking for?
Stories that are intended to entertain and delight first and foremost. Punchy, funny, warm-hearted texts, preferably featuring a great, kid-friendly main character. If funny isn't your thing, hopefully you've mastered beautifully spare and elegant writing. (Even straight-up nonfiction should be a joy and a pleasure to read!)
Hey, if the character has some entertaining experience and happens to learn a lesson or two organically along the way, fine, but the book should be about the EXPERIENCE -- if the resulting lessons overpower the story, that's a problem.
And if your query SAYS the book "is meant to teach a lesson about ______", that's a huge red flag that signals amateur.