I'd say it isn't easy relinquishing your story to someone else to show to the masses. Why do his non horror movies like the Green Mile and I think he did Shawshank get good actors?
I'd imagine it's precisely
because he relinquished control; studio knows he can't insist on crapping it up, like that campy 80s shit.
Stand By Me, Carrie, The Shining, Misery, Green Mile (probably others) all had established and/or talented directors sign on. Or they already had great screenplays, so they shopped solid directors.
Usually, once those two are set, A-List actors become interested. Or, the studio pursues them based on a promising product with a potentially lucrative return. Not sure with Shawshank, though, because Darabont was a first-time director. Guessing the script was sweet, which attracted the big names, but that's just speculation.