If there are all these books around that knock HP into a cocked hat, why don't they? Why aren't THEY at the top of the bestseller list? Kids books are often a matter of word-of-mouth, so if something is really good, it will get bought whether or not it's marketed well.
HP is at the top of the bestseller list because it's a good read. It's not Dostoyevsky or anything, but it's a fun read.
People can be sooooo jealous of success.