Because (and this is not to excuse them, but explain why) they are names. If you look at investors, they are usually narrow minded at the best of times and want "bankable" stars, someone who can love it up on Jonathon Ross and people can talk about,
Meryl Streep is a big name who will do lots of pr and publicity if the money is right .... so they paid.
If you consider the show has been going for 9 years and has numerous cast changes in the uk plus all the other actresses who have played the part internationally then you would think they could choose someone who can sing the part, WRONG they need that added bonus of someone you have heard of and has a large fanbase especially when the director has never made a film before.
Hollywood breeRAB its own egos