In addition to what Gonk said, surround sound plugins and support for studio software - such as Digidesign, Logic etc - didn't really come into their own until the last 5 years. Studio's don't always update their software to the latest releases either because they don't want to deal with bugs when everything is flowing well.

The surround sound software is much easier to use today and is much more automated, but it doesn't mean it is used and remixing a film's audio tracks would be a serious undertaking.