It can be a very daunting prospect when you're making a sequel to much loved classic and plan to introduce new characters that will then 'carry the torch' for future installments of the franchise.
The audience already has a connection with characters from the original and that you can't fake with new ones. It's something you have to earn and in order to that, the right casting is crucial. Of the three new characters, Rey was the most crucial and the filmmakers nailed it by casting then unknown, Daisy Ridley. As this newly released audition clip shows, she really didn't give them any other choice.