That's pretty obvious question to me but I didn't find an answer.
Saint-Petesburg was closer, it had better infrastructure, moreover it was the actual CAPITAL of Russian Empire.
But despite of these facts Napoleon had decided to take Moscow, it sure was a big city, but imperial government wasn't there.
While you're waiting on an answer, you might find my previous post on the 1812 campaign interesting. Briefly, the campaign was targeted neither at St. Petersburg nor Moscow, but the Russian army. Napoleon would have liked to have destroyed it in Poland and never bothered with Moscow, but the Russians retreated towards Moscow, so that's where Napoleon went too.