Actually, if you’ve watched TNG’s early seasons, you’ve already heard this unused theme, in bits and pieces.
The very first piece of music heard in TNG ever (that isn’t the intro itself) is a slower, less energetic (more introspective I guess?) version of that unused theme.
The main leitmotif itself appears all over early TNG.
I figure that, when Maccarthy was writing the early filler music, he assumed that his theme would become the main theme, and so heavily leaned into it when writing the filler music. Obviously, the main theme was tossed - but not his other music.