Sounds like you've been simming many years and have been letting your stats degrade. The original ridiculous AI decision that sent you to AAA was a temporary problem. As others suggested, it was probably down to an injury to another player and you being the only one with options. It shouldn't happen I agree.
However, I believe that if you had played and dominated AAA while continuing to improve your stats, you would have been right back in the majors pretty soon (or at least by next season).
I can't say for sure what's gone on since the original demotion but it seems like the player's overall, perception and production weren't good enough while you were simming so it never forced the team to make a spot for you.
That being said, SCEA really need to work on their manager/GM AI. There's no question about that. They tend to spend too much time on the new fancy things that don't matter much and never bother to fix the backbone of the game. I guess "improved AI" isn't sexy enough to sell a baseball game. They'd rather give us a Bowman Scouting Day that's over in 5 minutes.