It is generally considered that the Slavicization of Bulgaria finished under khan Boris. Now, Christianization certainly played a part in Slavicization since Boris invited the so-called Slavonic mission into Bulgaria and they spread Old Church Slavonic throughout the land and made it an official Church language. Boris wanted to make the Bulgarian Church as independent as possible from Constantinople so promoting Old Church Slavonic as a liturgical language instead of Greek was something Boris certainly felt was a good move. Thing is, many Slavs were already living inside the Bulgarian state and they outnumbered the Bulgars so it made sense to further integrate the Slavs into Bulgaria by promoting their language. At the same time, Boris was weakening the old Bulgar elite which rebelled against him and he retaliated by killing many influential boyars and their families. So, khan Boris must have considered the Slavs more loyal than the Bulgars, thus it would make even more sense to Slavicize his realm. By Slavicizing Bulgaria, he was strengthening his position.
1. Boris's desire for full concentration and centralization of power in the hands of the ruler, already as God's deputy on earth and unification of the Slavic and other population in a unified Bulgarian nationality
2. Boris's desire to be equal and recognized by Rome and Constantinople and the understanding that this is achieved by building one's own cultural identity.
3. The Glagolitic alphabet, created for the Slavs already made in Christian Bulgaria, contributes a great deal to point 2. There is no need for Bulgarians to write in Greek or Latin.
4. The sum of points 1, 2 and 3 leads to the fact that the 893th year of a common national assembly, the Slavic language in Bulgaria was declared as the official state language, and the capital of Bulgaria was moved from Pliska to Preslav. Boris is officially retiring and handing over the government to his third son, Simeon.
5. During the reign of Simeon was the greatest flourishing and development of ancient Bulgarian culture, literacy and literacy, which established and spread Slavic literacy in the Slavic countries