Why was Sumerian first mistaken for a special way of writing Akkadian?

Sumerian was initially mistaken for a special way of writing Akkadian because, following the decipherment of cuneiform, scholars first became familiar with the Semitic Akkadian language that dominated the texts found in Mesopotamia.12

Because the Akkadians had adopted the Sumerian writing system and adapted it to their own language—retaining Sumerian logograms while pronouncing them as Akkadian words—the script was viewed as a single, unified system. It was not until scholars gained a deeper understanding of the texts that they realized they were encountering a distinct, unrelated language with a unique structure. Even after its independent character was established, the interpretation of Sumerian remained difficult, and it was only through the study of bilingual texts and grammatical lists—originally compiled by ancient scribes to preserve Sumerian as a cult idiom—that the language was fully recognized and deciphered.132