Or HaChaim on Deuteronomy 9:6/אור החיים על דברים ט׳:ו׳

1וידעת כי לא בצדקתך….זכור אל תשכח, "Know therefore that not because of your righteousness …Remember and do not forget!" What did Moses hope to accomplish by admonishing the Israelites about their having had stiff-necked wicked people amongst them in the past? Perhaps their present righteousness would suffice to let them inherit the land and even to be able to point to their righteousness as the reason for this. Moses' reason may have been that the people reflected that even at the times they had been rebellious this did not result in their march towards the Holy Land coming to a halt. They had explained this anomaly to themselves by reasoning that G'd had to fulfil His oath to the patriarchs. Moses therefore wanted to make it plain that their own righteousness was essential in bringing about fulfilment of that oath. If they were to backslide they could still forfeit the promise to inherit the land.