Yes, there have been quite a few films that have been taken back into the recording studio and remixed for 5.1. In the end, it comes down to the availability of the original audio elements and the potential profit. In order to remix a soundtrack for 5.1, or in any number of channels other than the original sound mix, you have to have the origiinal audio elements available. It is rather impossible to take an exisiting mono mix, for example, and somehow only extract the various sound effects you want and them re-record them into the surrounds. As you can imagine, finding and perhaps restoring old sound elements and then remixing them is an expensive project.
Image thinks so with Dances With Wolves.
Each studio makes their own decisions in this matter, and it is all about profit in most cases. As for Rocky, there is no way to know if the original audio stems exist unless MGM were to feel that it would be able to resell enough copies of the film on DVD to warrant investigating its archives. I talked with an MGM rep, and the only sound mix at the time of the Rocky DVD release was a 2.0 stereo remix. Bear in mind that even when interest exists to remix an old soundtrack, film preservation is not what it should be. Take the re-release of Star Wars.