I believe MCA still owns the rights to it. So they were unable to do a remastered version of it in 2003.
Hopefully another company is able to full expand it in the future. Along with many of the Roger Moore Bond scores that were not expanded in 2003.
The Man With The Golden Gun, The Spy Who Loved Me, Moonraker, Octopussy and A View To A Kill
I thought the reason some of those other soundtracks weren't remastered was because the master tapes haven't been found. It's pretty impossible to re-master something that doesn't have an available master! 
That was the reason given, but the definition of the word "missing" is a bit misleading.
The tapes are out there. They just have to look for them first. From what I understand this was not done.
Oh and by the way they did have The Man With The Golden Gun full score, but chose not to remaster the full score due to time and budget constraints of the project in 2003.
Lukas Kendall has commented specifically on the Golden Gun score.
I think at times rather than answering questions on why, it's easier to say something is missing. When in truth it is out there, it has to be looked for first. Of course there has to be time and $ spent to finish a project as well. Every project has budget and time constraints.
So it was just easy to use what they had and not bother to look for what was needed for the others.
AKA the "other" scores are "missing"...
BTW. This has all been discussed here on the forums before.