Here's why:
* The invitation was only for the royal family and nobility. Cinderella was a servant girl, and thus, not included on the guest list.
* She was able to attend thanks to her fairy godmother. The fairy godmother transformed her into a beautiful princess and gave her a magical carriage to attend the ball.
So, while Cinderella didn't receive an official invitation, she was able to attend thanks to a little bit of magic!