They're totally able to make a MKX for last gen, that's the point.
I'm sure the MK9/Injustice engine didn't go anywhere, so it's not like they have to create an entire new engine to put this game to work on older systems. As MK9 showed (I haven't played Injustice), you can have a good game with relevant gameplay, the only thing they would have to do is "skin" MKX upon that foundation. I'm not sure about the variations, but let's say you have to take them out... well, you still have a competent game for those who can't afford the bigger version. The gap between PS3/360 and PS4/X1 these days isn't as huge as it was from, let's say, SNES to PSX. I'm pretty sure that as long as High Voltage actually works on it, the last gen MKX won't be bad.
Huge numbers or not, the fact is that the older systems are still around and someone (seems like it was Warner) decided that it was worth creating a version for them - after all, as I mentioned above, it's not like they're not able to do it. I just expect them to be professionals and give us some information about the game. It'll be awful if it is delayed, yes, but I won't have the means to buy a PS4 until June, so it doesn't change much of my situation. I just want some information until then, in order to hide in a cave to avoid spoilers and wait for the game.