Because deep inside Miranda, even though she’s very satisfied with the job she has, is not happy.

"Often it’s fun to play a character which is difficult to like, but this time it wasn’t fun for me. Because deep inside Miranda, even though she’s very satisfied with the job she has, is not happy. This obsession for fashion, which makes her consider herself very elitist, needed to be put aside and shown as a shallow and sterile in the long run. She refuses to admit that all of it is actually irreparably superficial because that would question all her power, that would prove the vacuity of her life, that would lead to fatal consciousness of how mislead her life is. Her behaviour seems strange and that’s why I don’t think she’s a monster. What’s more, to me it seemed impossible to get into her skin and do the justice as I would do it as an actress, without giving her a further right of appeal." -Meryl Streep on playing Miranda Priestly