I have theory that NRS might do it because they originally planned on doing a fighter pack 0 for INJ2 where we'd get Batgirl, Zod, Shazam, and Doomsday.
It is past the 25th anniversary of MK so to keep the fire going, they definitely "aren't done yet, not even klose" to quote Ed. And since they planned on Mileena and Rain from the beginning, then it is safe to assume that the guest was changed to Rambo as a result of not getting Ashe.
Finally, when you release 3 characters, that's 3 more characters to balance. As we've seen with the balance patches for the game, they only have come for HUGE changes, and what's a big change? The last 5 characters.