Final answer:
The Harappan culture is mysterious to historians because they haven't been able to decipher Harappan writing, limiting our understanding of their language, literature, and historical records.
Step-by-step explanation:
The Harappan culture remains mysterious to historians in terms of Harappan writing. The writing system of the Indus Valley civilization is yet to be deciphered, which limits our understanding of their language, literature, and historical records. As a result, we have little knowledge about specific aspects of their society, such as religious practices, political systems, and artistic themes that might be revealed through their written documents.