They didn't need a cast of stars, they just needed people who could act and they got exactly that.

Given the size of the show and the multi-season commitment it got from Amazon, it would probably take an actor like 7 or 8 months of the year to film an entire season and big stars probably don't want to dedicate years or their careers to a show like that.

Just look at Henry Cavill, he binned The Witcher pretty much so he had time to do new Superman movies.