Suppose why a Byzantine historian tells about the early history of the Slavs?

This is due to the fact that the Slavs did not have chroniclers back in those days. The most important chronicle of the history of Kievan Rus is “The Tale of Bygone Years”. It was created in the 11th century. So it covers the events since the emergence of the state of the Eastern Slavs, that is, since 862, when the Varangians arrived on the lands of the Novgorodians together with Rurik, who became the founder of the first princely dynasty.