It’s most likely because they were fully human who were adapted to their environments.
It’s interesting that the gene sequence on chromosome 9 that produces blood type A differs from the sequence that produces B by less than two dozen base pairs. O which doesn’t code for a blood protein is nearly identical with A, merely lacking the first base pair, making the successive sequence non-functional.
Let’s not dive into the M-N blood group, but the wikipedia page on blood types at least used to be magesterial in its discussion of the entire topic.
Neandertals and Denisovans are humans. They are more different race, than different species, as proven by evidence of interbreeding.