The answer is quite simple.
Publishers don’t like to take a chance on anything too different from what they have already published. ( See below: ‘Too radical of a departure from traditional juvenile literature.’)
As one publisher famously said, “We want the same thing … only different.”
Acceptance criteria for large publishing houses are based almost exclusively on what they have sold before. All the necessary components of calculating potential profits — cost/benefit analyses, market projections, etc. — rest entirely on previous sales, not on the inherent value of the manuscript.
Sometimes we forget that publishing is a business like any other. Businesses don’t like to take chances, and the bigger the business, the more cautious it will be. (Random House turned down the Harry Potter books because they were “too long, and nobody would read them.”)
These pointed— and completely misguided — rejections should not only encourage you on your path to publication (you are in good company!), they should steel you against the frustration of the inevitable “Thanks, but no thanks.” Imagine where we would be if L. Frank Baum (or Oscar Wilde!) had quit after being given the brush-off. They didn’t quit and neither should you.
(Just as a case in point, my trilogy was rejected repeatedly for five years before being picked up by Random House.)
But don't take my word for it; here’s what publishers had to say about the following books:
The Picture of Dorian Gray by Oscar Wilde
'for your own sake do not publish this book.
Garfield by Jim Davis
'Too many animals, and cats don’t sell.'
Zane Grey (93 books, 21 films)
'You have no business being a writer and should give up.'