Firstly, one only has to look at a map of the positions of the soviet civil war/pre 1939. Petrograd was mere miles away from first the German occupied areas of Russia signed away by the bolsheviks, and then also threatened by the breakaway Baltic republics and Finland. Moscow, being in the centre of Bolshevik Russia was a much more defensible position

Secondly, Moscow and st Petersburg have had a sort of duelling cultural meaning in Russian culture. St Petersburg was the city of the tsars and represented, essentially, westernism. Moscow was the cultural heartland of Russia. In picking Moscow, the Bolshevik in part rejected the capitalist west to build a new society out of true Russia