I don't know how unpopular this is, but I don't think I've ever seen anyone else talk about it online, so fuck it:
It's weirdly common for video game soundtracks to feature the stuff the devs/audio production guy liked the best and omit the music that was most prominent in the game. They'll have the end-credits song that everyone hears once, but leave out things like mission select menu or hub world music that you listen to for ten hours straight.
I suspect their logic is that those tracks are designed not to draw too much attention to themselves and just set an atmosphere, but THAT'S the music you hear when you actually play the damn game. I had no clue what the main theme of Metal Gear Solid was and I'm not sure it's ever played in-game (maybe once?), but I'll remember this track for the rest of my life.