1. Because for an average kid from target audience Songhai, Siam, Zulu and Ethiopia are even more fantastic and out-of-this-world than Zergs and Protoss' - they have no clue and they don't care, whereas old fans would go "ooh a new thing! Great!" so that would stall them long enough to not see the shortcomings of the release.
2. So that Zulu, Ethiopia and "The Danes" can be sold separately later on. I mean, Ski Infantry? Bring them on, what a splendid idea!