I think it was mostly because it was a simple narrative with an identifiable protagnonist. - Edit 1
Before modification by Chas at 13/07/2010 11:59:23 PM
After all, it's possibly the best selling children's series of all time, adults enjoy it too, and it has won numerous medals.
Probably the biggest selling point of that is the school setting. That's something that a large number of people are familiar with, especially since Rowling kept everything to the basics and didn't overindulge in the magically aspect of it.