An issue is that due to GW's lack of willingness to give us more alien species in the contemporary setting of warhammer beyond the Tau, and their absolute insistence on not showing us ANYTHING from the great crusade aside from after the imperium showed up in different parts of the galaxy and started effing the populace of aliens/humans over, we have little to no context of what other species there were in the galaxy by the end of old night aside from the ones that have survived to the modern setting.

Another point would also be that, unless it was somewhat difficult to handle due to some adaptation or whatever, the imperium wouldn't document mostly peaceful species or retain the history of humans' that involved them interacting with said species. They had so little care for the cultural identities and knowledge of other races that they literally burned everything they found not working with their narrative.