The same reason everyone here gets so upset about anything having to do with *NSync or Britney, etc.
I'm not going to give a positive or negative view about Harry Potter, but there's no denying that at this point it is nothing more than a pop culture phenomenon.
There's nothing wrong with Harry Potter and there's nothing wrong with liking Harry Potter, but when compared to Tolkien's work it is much lighter. That's fine. Here's the analogy I use to describe them:
Harry Potter = Mickey Blue Eyes
Lord of the Rings = The Godfather
Both good, but in different ways (and listen people, if you hated Mickey Blue Eyes keep it to yourself, you should at least understand the point).
I think a lot of LOTR fans are just a little miffed that a creation of the last couple years and has hardly stood anything resembling the "test of time" is getting so much hype.