Why is the study of mapmaking called topology?

It isn't. You are thinking of topography, the study of surface geographical features. The study of map making as such is cartography.

The etymology of the Word topology is from the Greek Words, tópos, meaning place, and lógos, meaning reason and discourse. Hence tópos-lógos --> topology --> place-reasoning. From tópos we also get the Word topical and from lógos, logic.