As opposed to the ancient Greek, Slavic mythology did not leave any written documents behind. Why?

Slavic peoples did not use writing before Christianisation. Due to “illiteracy”, Slavic believes, and myths together with them, continued to be passed down orally after Christianisation and the beginning of literacy and in time have disappeared “into oblivion”.

Up to today, little has been preserved with the people. At best, we have heard of Perun, Svarog or Veles. We see the word Triglav as a toponym and we do not know that Triglav was a three-part Slavic god who unified three roles of beings. The realistic one, the unrealistic and the right one.