I was exaggerating a little bit. Bulgarian lost many  cases, so article were introduced. Other Slavic languages have 7 cases (Russian 6 cases), so the articles are redundant.  It’s wicked to see Bulgarians are having fewer (3 cases)? Old Church Slavonic used in Rus and later in Russia , Ukraine was modified to suit eastern Slavc speeches. Phonetics, orthography, morphology was modified. Modern Russian is most influenced by OCS in comparison to Belarusian and Ukrainian. Especially written Russian.