Why wouldn't BigCo offer to buy rights under a different license? I'm an outsider to this kind of deal, but it seems to me like a good way for both parties to g

Why wouldn't BigCo offer to buy rights under a different license? I'm an outsider to this kind of deal, but it seems to me like a good way for both parties to get something they want -- open source projects being often short of funding. The obvious case where that wouldn't work is a project like Linux with many contributors and no copyright assignment. But others like Clojure organize themselves differently.