I imagine that you have to think of any big production as part of a very well established camera ecosystem. The camera, lens, whether you use film or video etc can not be isolated choices. Arri, which has been around since the silent era, very purposely made the Alexa so that it can be interchanged with (film-based) Arricams almost without notice - all the bits of gear that make it work (gears, flags, matte boxes) are widely availabe and second nature to a practised camera crew.
You can find crews around the world that will know how to work such standard 35mm-like gear. While RED has made tremendous progress, its layout, unique setup, and the depth of support by rental houses are just not as well established. While very technically adept directors like Fincher and Jackson might prefer a RED and enjoy fidling with it and pushing the camera system to its limits, most directors have enough on their hands and want as little drama as possible from the camera dept.