I don't keep up with many blogs but came upon one from a literary agent saying why they don't like to work with picture book writers. The gist was that there is not much money in it for anyone and everyone is always trying to do the same old thing over again.
My thought is that the same themes come up not because people think it is an easy sell but because there is a demand. Princesses, trucks, pirate, dinosaurs, and space are themes that have been around for a very long time, the trick is to do it different. My son asked me a question based on a picture he saw of a rocket and it generated some research on books about space for the youngest of readers. Harry and Horsie and Spaceboy are two of our favorites, I love the illustrations in both and the stories are fun too. I was really surprised to see that Jan Brett had sent hedgie into space. I know people in the industry may be uttering the words "if I see one more submission about... I am going to scream" but as a parent of two children who love picture books and are always on the lookout for something new, I say keep 'em coming.