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Why did Ruthenian remain important in the Grand Duchy of Lithuania before Polish replaced it in administration?

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Map showing Poland's territory in 1634 under the reign of Wladyslaw IV Vasa, shaded in pink and spanning from the Baltic Sea in the north to near Kyiv in the southeast. Major present-day cities such as Warsaw, Krakow, Gdansk, Vilnius, Minsk, Lwow (Lviv), Pinsk, and Kyiv are marked within the territory, with surrounding cities like Moscow, Riga, Vienna, and Budapest shown for reference. — Polish-Lithuanian Commonwealth Poland's territory in 1634, during the reign of Władysław IV Vasa.

Ruthenian remained important in the Grand Duchy of Lithuania because it served as the de facto official language for administrative and legal purposes for much of the state's history.1

Several factors contributed to this prominence:

Demographic Composition: As the Grand Duchy expanded into vast East Slavic territories, Ruthenians—the ancestors of modern Belarusians and Ukrainians—eventually constituted the largest ethnic group in the state.1
Administrative Integration: The Lithuanian leadership reorganized its government, military, and legal systems based on Ruthenian models. This reliance on Slavic customs necessitated the use of the Ruthenian language in official state affairs.12
Cultural Assimilation: Early Lithuanian rulers and nobility often assimilated into the local culture, adopting the Orthodox faith and utilizing the language that had developed from the traditions of the medieval Rus.21

This status persisted until the influence of the Roman Catholic Church and the Polish language grew among the nobility following the dynastic union with Poland. By the time Polish officially replaced Ruthenian in 1697, it was merely a formal recognition of a long-standing shift in the preferences of the upper classes.1