Another question--is it the voice that distinguishes the two (commercial vs. literary)? Or is it everything together--voice, subject matter, amount of description, etc.? (And actually, I find the commercial stuff harder to write, like the series that need to be short, tight, and often with short deadlines. It's a joy to write more literary stuff where it can be as long as it needs to be to tell the story.)Commercialness is about popularity, and that means at the least that the language is not difficult for most children to get into. Outside of that, what makes a book popular can vary a great deal.
And don't think that there are only two categories. There are plenty of books that are neither particularly literary nor commercial. They're often called "science fiction".