1. Napoleon's goal was to pursue and defeat the Russian army in the field, not necessarily capture cities. Going north would have meant releasing the pressure on the main russian field armies and let them engage his main force at their discretion while exposing his flank.
2. St. Petersburg remained the political and economic capital of Russia; St. Petersburg never displaced Moscow in real world importance.
3. St. Petersburg was shielded from land and sea with prepared fortifications on both and Napoleon lacked a fleet to effectively blockade it.
4. Its in the middle of a very dense forest and swamp, not the the best logistics and ability to maintain a siege.
The Nazis made the same choice in WWII and even though they were able to control the Baltics and had Finland as an ally never seriously threatened to take the city.
1. Napoleon's goal was to pursue and defeat the Russian army in the field, not necessarily capture cities. Going north would have meant releasing the pressure on the main russian field armies and let them engage his main force at their discretion while exposing his flank.
2. St. Petersburg remained the political and economic capital of Russia; St. Petersburg never displaced Moscow in real world importance.
3. St. Petersburg was shielded from land and sea with prepared fortifications on both and Napoleon lacked a fleet to effectively blockade it.
4. Its in the middle of a very dense forest and swamp, not the the best logistics and ability to maintain a siege.
The Nazis made the same choice in WWII and even though they were able to control the Baltics and had Finland as an ally never seriously threatened to take the city.