Because:
1. The book "Ella Enchanted" was published in 1997, (10 years before the release of Disney's "Enchanted".
2. The movie "Ella Enchanted" was released in 2004, (as you already said, 3 years before the release of Disney's Enchanted)
3. How where they to predict that in 3 years time "Enchanted" was even going to be made?
4. But they are both very different movie, except that they follow a modern fairy tale edge.

Yeah and Enchanted is 100% better than it

But Ella Enchanted came first. I'm sure they didn't plan it that way. Plus, they wanted to keep it with the same name as the book, which was, as mentioned, published before they were even thinking about Enchanted.