Kutuzov is not destined for greatness. Instead, as Tolstoy tells us, history has him down as a lazy, butt-kissing coward. To Tolstoy, this is as good a piece of evidence as any that Kutuzov was actually awesome.
Another piece of evidence is that Kutuzov did nothing out of self-serving motives. He just had one aim: to get the French out.
A final piece of evidence is Tolstoy’s sense that Kutuzov really had his finger on the pulse of the Russian people and was able to use that to get the French out ASAP. Or something. The description gets a little mystical and mysterious here.
To sum up, because he was so effective and not out for himself, Kutuzov doesn’t fit the traditional stereotype of the “great man.” Instead he’s remembered as an old buffoon.