Likely because Mendel practiced actual science and his work wasn't intertwined with racism, eugenics, Sanger and that ilk.
Last time I checked, Darwin applied actual scientific observation and study to come up with his theory, and it contained nothing of "eugenics, Sanger, and that ilk". Charles Darwin believed in inheritance through a theory of pangenesis, not genetics.
Mendel was the father of genetics (which Darwin had absolutely NO knowledge of), so when the fundamentalists here try to blame NAZI eugenics on "Darwinism" it's like trying to blame Tiger Woods sex scandal on steroid use in baseball players. Just as the latter two had nothing in common besides both being sports, the NAZI eugenics program had nothing in common with "Darwinism" aside from the fact they both cited "science". You can blame "Darwin" for eugenics as much as you could blame Copernicus or Marie Curie.
If they want to blame "science" for the holocaust, they could start by figuring out what type of science was corrupted to create the theory of eugenics, and of course the answer would be Mendellian genetics.