Why are people so vehement about denying Blacks their historical accomplishments?

I am a historical researcher and I have noticed that other racial groups over the last 40 years have been given their "dues" in contributing to civilization. I.E the aztec and chinese achievements were supposed to be a mystery to the modern world & there was even speculation that aliens built their civilization. Until recently they were not credited with their accomplishments. But Blacks on the other hand have been vehemently denied their civilization's achievement's all the way from Egypt to Zimbabwe. Note: Before the 17 and 1800's Egyptians(in the earlier part of their kindgdom before the Ptolemies) , Moors, Nubians and the early Ethiopians were considered black africans i.e shakespeare, herodotus, etc etc all ancient historians gave detailed accounts. But now even with overwhelming evidence people are slow to give them credit. Why is this? I wonder that now that we alll have our cake why can't they have theirs as well