I think they've gotten themselves in a pickle; they are simultaneously choosing to start with 18 civs in vanilla (same as Civ V) and adding more unique content per civ, but have to live with the expectation from people of having even more civs at the start because they are "needed".
American, Aztec, Chinese, Egyptian, English, French, German, Greek, Indian, Roman, and Russian civs have been in every game in the franchise at release, and by rule should always be expected to do so. That knocks down 11 of 18 starting slots. Japan wasn't in the first game but has been in every vanilla since Civ2 so that eats up slot number 12. Then they'd want to have at least a few new civs for the new game's vanilla release. It gets crowded quite quick.
What's interesting is that the veteran civs which take next priority (having been in every major installment after DLC is counted) include Babylon, Mongols, and Zulu... three civs which appear to be absent form our game of bingo. It's disappointing if they aren't in at the start but I doubt that they're getting dropped for good given their track record in attendance.