Here's why:
* Early Influences: Muscovite literature emerged in the 14th and 15th centuries, during the period of the Grand Duchy of Moscow. It drew heavily on the literary and religious traditions of the Eastern Orthodox Church, which used Old Church Slavonic as its liturgical language.
* Common Roots: Old East Slavic was the language spoken in the Eastern Slavic lands (which included modern-day Russia, Ukraine, and Belarus) before the 14th century. It formed the basis for later literary languages like Russian, Ukrainian, and Belarusian.
* Religious Influence: The Church played a crucial role in the development of early Muscovite literature. Monks and scribes produced religious texts, chronicles, and hagiographies in Old Church Slavonic, laying the groundwork for subsequent literary works.
* Evolution and Adaptation: Over time, Muscovite literature adapted and evolved, incorporating elements of the vernacular spoken language and incorporating new themes and genres.
Therefore, while Muscovite literature eventually developed into modern Russian literature, its roots firmly lie in the rich tradition of Old East Slavic and Old Church Slavonic literature.