Why? Ultimately it was his campaign that lead to Napleon's downfall.
He had very few tactical successes. He wasn't in charge of the early phases of the war of 1812 (which were also carried out with great discipline and accomplishment). He died before the fighting of 1814 started in earnest.
His great achievement was being well-liked and saving the Russian army's morale in giving a battle when constant retreat created tensions with the officers and soldiers, and in knowing that a retreat afterwards was still necessary.
But arguably he wasn't even the greatest coalition-era Russian general, let alone the greatest coalition-era general, let alone among the greatest generals of all time.