Why did Fox screw us on the DVD?

I don't get it. When the VHS release of all four ALIEN films came out, this incredible documentary was included as a bonus tape in the set. But when the films first came to DVD, instead of simply including it in the set, it was a premium you had to send away for. (While supplies lasted...and they didn't last long.) Even the expanded ALIEN QUADRILOGY boxed set didn't include this. (Although a few advance online listings for that set did erroneously include it in the contents list.)