Rocky Balboa flashback scene with Mickey Goldmill

In the movie Rocky Balboa (2006) there is a scene where Rocky visits the old gym where he trained for his fight with Apollo Creed. In this scene he has a flashback of Mickey Goldmill giving him a pep talk. Obviously this scene was not filmed for Rocky Balboa as Burgess Meredith passed away in 1997 and on IMDb he is credited in the film for "archive footage" which leads me to the conclusion that this scene was a deleted scene from Rocky or Rocky II. It is a very touching scene and works perfectly as a flashback.

Now to my question: Why wasn't this scene included in the original movie? I understand that is kind of difficult one to answer unless you ask the producers themselves. And that is really what I'm looking for here. I've been trying to find some sort of commentary about the scene from the film makers to explain where the footage came from and why it wasn't used previously.

Here is a link to the scene: