God, the Great Scientist was experimenting with various Hominids, such as Homo erectus, Homo habilis, Homo ergaster, Homo heidelbergensis, Homo neanderthalensis etc. long before He finally created Homo Sapiens, using gene sequences which worked towards His purpose, placing these in Homo sapiens. Thus it LOOKS like sapiens mated with neanderthals, but I doubt that.
I can speculate why God never offered the Neanderthals salvation, and why there needed to be one last iteration of hominid after Homo neanderthalensis.
The Neanderthals had bigger brains and stronger muscles than us, and there was less body dimorphism between the sexes. Even Mrs. neanderthalensis could go out and club a giant sloth or spear a mammoth.
God wanted dumber, weaker, less capable, more sheep-like children. Because Neanderthals were so capable, they were less sociable, and less dependent on one another and society. Thus, due to our weakness and patheticness, we are more dependent and needy, including needy for a relationship with God.
That is why it is hard even today for a rich man to be saved. The rich are not needy. Blessed are the poor in spirit, for theirs is the kingdom of God, which means Homo sapiens.
All the super-hero movies which glorify human strength (Hulk, Hercules, Thor, Superman, Ironman etc) glorify an independent, self-capable human - which is a laugh, because if they had to fall, or be thrown around like they are in the movies, they would be whining like - well - we do.