Write an article that answers the following question: Why did Aleksandr Miasnikyan replace Gevork Atarbekyan?
On December 4, 1920, when the Red Army entered Yerevan, the government of the First Republic of Armenia effectively surrendered. On December 5 the Armenian Revolutionary Committee , made up of mostly Armenians from Azerbaijan, also entered the city. Finally, on the following day, December 6, Felix Dzerzhinsky's dreaded secret police, the Cheka, entered Yerevan, thus effectively ending all existence of the First Republic of Armenia. The Armenian Soviet Socialist Republic was then proclaimed, under the leadership of Gevork Atarbekyan. On February 18, 1921, a national revolt against Bolsheviks started. Gen. Garegin Nzhdeh, commander Garo Sasouni and the last Prime Minister of independent Armenia Simon Vratsyan took the lead of the anti-Bolshevik rebellion and forced out the Bolsheviks from Yerevan and other places. By April the Red Army reconquered most part of Armenia. However, Atarbekyan was dismissed and Aleksandr Miasnikyan, an Armenian high-ranking Red Army commander, replaced him. Garegin Nzhdeh left the Zangezur mountains after the Sovietization of Armenia was finalized in July 1921, leaving Azeri-populated villages cleansed of their population. Persuaded by Soviet leadership, Zangezur had already been ceded by Azerbaijan to Armenia in November 1920 as a "symbol of friendship".