As a disclaimer, I work on French colonialism in North Africa and the Pacific, and am more broadly trained in comparative empires, world history, and European history.

The history of Sub-Saharan Africa is certainly one neglected field. There's a few reasons for this. Above all, African history is plagued by the question of sources. Although some people in Sub-Saharan Africa, say those on the Swahili coast and what is now Mali, have old written records, many others do not. As such, historians are forced to rely on anthropological, linguistic, and biological (including genetic) evidence, as well as sources like art. Colonial records are used, but there are questions of reliability. The British, French, Germans, Belgians, etc. did not really care a ton about documenting African histories and they arrived on the continent with a number of preconceptions that lead them in directions that are difficult to get past (for example, the "tribe" as a discrete group, the idea that Africa has not changed and was still "prehistoric" when they arrived, etc.

Because the whole field is so theoretically and methodologically difficult, texts on Sub-Saharan Africa can be really difficult to read. This prevents the history of Africa from drifting into the public mind, by and large.

Besides that, another case is Central Asian history (our famous "-stans"). This is more often a linguistic issue, as there is a ton of literature on that region in Russian, but little of it gets translated to English. Moreover, Anglophone historians don't tend to research it nearly as much (although some institutions like Indiana University produce a voluminous amount of knowledge in the region).

Surely there are others, but these are the first two regions that come to mind for me.