I think that Freud's "wounding blows" explanation is good:
"In the course of centuries the naive self-love of men has had to submit to two major blows at the hands of science. The first was when they learnt that our earth was not the center of the universe but only a tiny fragment of a cosmic system of scarcely imaginable vastness. This is associated in our minds with the name of Copernicus, though something similar had already been asserted by Alexandrian science. The second blow fell when biological research destroyed manís supposedly privileged place in creation and proved his descent from the animal kingdom and his ineradicable animal nature. This revaluation has been accomplished in our own days by Darwin, Wallace and their predecessors, though not without the most violent contemporary opposition. But human megalomania will have suffered its third and most wounding blow from the psychological research of the present time which seeks to prove to the ego that it is not even master in its own house, but must content itself with scanty information of what is going on unconsciously in the mind."
But the resistance to Darwin is stronger than that to heliocentrism or pychoanalysis so you need an additional explanation. I think Becker's Denial of Death can provide it, we often disavow what reminds us of our mortality and being a mortal animal is such a reminder.