There are several reasons for this. One is that the inhabitants of Great Zimbabwe apparently abandoned the place, perhaps as early as the 1500s, certainly by about 1700. No one knows for sure why they left or where they went. They left no written records, for they had no written language. The second reason this place has remained shrouded in mystery is due largely to racial prejudice.
Racism is actually a rather modern sickness, but an insidious one.
It is largely an outgrowth of the trans-Atlantic slave trade that destroyed a great part of the fabric of African culture. White conquerors of black Africa came to regard Africans at best as helpless children and at worst, and more often, as an inferior species of savages in order to justify taking their land from them and them from their land. Whites became so blinded by prejudice that, rather than accept that blacks had a history of their own, they fabricated a history that fit their beliefs. Probably no other prehistoric site has given rise to such strong, widespread and often emotional responses to questions about its birth, life and death as Great Zimbabwe. It may have been incomprehensible for me to accept that this isolated place in the African interior was a complex social, economical and historical capital of a highly centralized, wealthy state.