For now, and probably forever we will never know. All we can do is speculate whether if they already had made the Mongols Civ along with the Babylon Civ at the same time with all the other Civs, and decided to maybe sell the latter later, or not.
I can think that maybe they went along with giving us the Mongols free to pacify us, or some of us, to think that they are still doing whatever they can to make this game better while we wait. Kinda begs the question on how many things (they call it DLCs) they are withholding from us, and if they do have many other Civs and other soon to be called DLCs, proves the point that their scheme from the beginning is to milk more money out of us after the half-completed vanilla version.
Then don't purchase the DLC.
This argument has been used countless times for countless games. People still buy DLC. DLC continues to break records. The process rinses and repeats.
A collective of people complain about a game feeling unfinished, unpolished, empty, etc. Company then releases DLC. It's the new alternative to the expansion. Why? Expansions are risky.
Look at Titan Quest. A game that, just like many of you, people said was 'empty, buggy, incomplete.' They patch, patch, patch, people still feel this way. Suddenly: Expansion. People stop complaining about the game feeling empty, being buggy, feeling incomplete. Company goes under.
Why?
Expansions are risky. DLC is not.*
Even further, when a game has many expansions and a NEW game is released, players of the old content expect EVERY SINGLE DETAIL from the past game to still be in the new game. It's like EQ2. When EQ2 came out, EQ players said two things:
1. THE GAME IS INCOMPLETE (EQ had some 7 expansions by that point I believe). Compared to EQ at launch, EQ2 had just as much content.
2. THE GAME IS BUGGY (EQ had some 6 years to fix all of the bugs in the title). Compared to EQ at launch, EQ2 had just as many bugs.
But I digress. If you feel that Firaxis is undermining you, or you feel they did not deliver on their product, or whatever else, just don't buy any products from them. Period.
ETA: If you even, for a second, believe that Firaxis is some god game company, you're absolutely wrong. If nobody buys their next product they will go under.