Regarding BTT 90's, I'm guessing there is a licensing issue. I'm guessing this based on the fact that the few still-active players at the time the 80's game was released (Clemens, Rickey H, Larkin, Orozco, etc.) are designated by team and number rather than name. There would apparently be extra cost in using active players. Of course, there are a lot more active players in the 90's than the 80's--a whole lot more.
So I'm guessing that if we see a new decade league, it might be the 70's rather than the 90's. If the volume generated is sufficient to support higher licensing fees, we might then see a 90's version of the game. I might be wrong, but this is my guess.
An all-time greats version of the mystery league game would also avoid this licensing problem and would be really welcome.