Laura Mitchell Kelly? Kelli O'Hara? Liza O'Hare? Seriously? The producers would make more profit by throwing money in a trash can than they would if they cast any of those people as Eliza in a MY FAIR LADY movie.
They need a name, that is, someone who the public knows, and someone with a following. So your names are Keira Knightley, Jessica Biel, Natalie Portman, Anne Hathaway, Megan Fox, Emily Blunt, Kate Hudson, or even Leighton Meester. The movie needs to sell both domestically and internationally. It's a musical in the very traditional sense based on a Shaw play, it's already a hard sell as it is, why would the producers cast a nobody as the main role?
Expect:
Keira Knightley or Anne Hathaway as Eliza, Hugh Grant, Daniel-Day Lewis, George Clooney or Colin Firth as Higgins, Michael Cane as Mr.Doolittle, Emma Thompsom, Julie Andrews or Maggie Smith as Mrs.HIggins, and James Marsden or Hugh Dancy as Freddy. These are names, names sell.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"