I can start on a broad level: it goes back to Stardock just being Stardock, going back to the days of Elemental. You've gotten this feedback before: not fixing glaring issues, not responding to player feedback, but then going on and selling new enhancements in DLC. Adding a second story to a house with a cracked slab.
But for specifically this one, the real gameplay issue is not that there aren't enough species, but that your citizens for any single game are all alike. Recruiting citizens of other races is uncommon, and there is only minor variance in the stats. All you did was offer more & different ways for all your citizens to be all alike; same strengths & weaknesses. Meanwhile, entrepreneurs and the Altarian special citizens (empaths?) are still not worth much. Where's the strategy in that? If my non-xenophobic race could research Stellar Immigration and have a way to recruit Navigators to man my Tech capital, now things start to get interesting. Or if I ran low on food, but I could ally with a synthetic race and do a cultural exchange. There's more richness in gameplay to that.
Plus, the intended design of some species being slow-growth/others fast-growth isn't working now, and I don't see that changing in the pack. I can buy a couple recruiting stations in orbit right now, and pretty much populate the entire galaxy with that (even with no promethion). They grow a citizen every turn.
Stardock has been doing that for awhile now: not improving on the real issues and going off and doing something else (for a price). Plenty of other games have had highly unpopular expansions before, but this is the only line I can think of where the majority of expansions, I simply didn't want.
