One soundtrack that I still honestly can't believe was never made was an official release of Marvel vs Capcom 3/Ultimate Marvel vs Capcom 3. Capcom usually releases their stuff through the Suleputer record label, practically everything was put out by them except for a couple of albums. It's a real shame because the music in that game is brilliant, often with great remixes or original character/stage themes. There are so many great tunes in this game with personal favorites being Deadpool's Theme, Wesker's Theme, Ghostrider's Theme, Wolverine's Theme, among the many others I didn't mention as the whole thing was very well done. I am a big fan of fighting game soundtracks as they tend to have so many varied and interesting high energy tracks. They released Street Fighter IV's OST and even the newly released Street Fighter X Tekken is getting its OST (through the Japanese Limited Edition sadly). The only reason I could think of is that Marvel didn't authorize them to release it because they have theme's that they may/may not agree represent their characters properly. Not really sure of the exact reason, just guessing. I would still buy it even if it were to be released later as it's a great soundtrack!
