Why was Harry Potter so successful?

It's a simple question,
Why was the Harry Potter series so successful?

I mean, it didn't add anything new to the genre, and there are a lot of other better series, with better character development and what-not.

I'm not saying its bad, but what makes an almost average book so popular?

Rowling is REALLY GOOD at writing easy-to-read prose. I don't mean good prose, I don't mean complex or realistic or beautiful prose, I mean goes-down-fast prose. Harry Potter is the equivalent of cotton candy. You can eat a ton of it no problem.

All she really did was write a potboiler-level fantasy novel that people found really EASY to read, while just "hard" enough that they didn't feel too guilty reading something for children.