Or HaChaim on Deuteronomy 9:5/אור החיים על דברים ט׳:ה׳

1 לא בצדקתך, "Not because of your righteousness, etc." Moses meant that the Israelites' righteousness was not a reason for allowing them to dispossess other nations. This did not mean, however, that they were not a spiritually upright generation. Moses said something similar to the people in Deut. 4,4 when he attributed their being alive to their having cleaved to G'd. Such conduct does not, however, mean that other nations were to be killed on account of the Israelites' goodness. Had there not been a covenant with the patriarchs, G'd could not have justified expelling and killing the Canaanites on account of the Israelites. At the same time, the oath sworn by G'd to the patriarchs by itself would also not have resulted in the Israelites' victory over the Canaanites if they had not been wicked. We have a Mishnah in Eydiot 2,9 which states that "although a father confers upon his son certain attributes (genes) such as handsome appearance, intelligence, etc. etc., there is a limit to the effectiveness of such transfer of parental genes after a number of generations. The Mishnah bases this statement on Isaiah 41,4 קורא הדורות מראש, "He (G'd) determines ahead of time for how long the father can transmit his genes to successive generations." Raavad comments as follows on this statement. "G'd foresees whether a generation is deserving, and if so He allows these genes to be transmitted from father to son until a deserving generation arises such as the generation of Joshua. Thus far Raavad. From this we learn that in addition to G'd's promise to the fathers it is also essential that their descendants be worthy of that promise. Accordingly, Moses told the people that their righteousness was not enough by itself to bring about their conquest of the Holy Land. In fact their righteousness did not even help the oath to Abraham to be fulfilled. The only thing it was good for was to ensure that their conduct was no impediment to the good that G'd had promised being fulfilled now.

2בי ברשעת הגוים…ולמען הקים…אשר נשבע, "for due to the wickedness of those nations…and in order to keep the oath which G'd swore, etc." None of these reasons would have sufficed by itself. Had the Canaanite nations not been wicked G'd would not have had a legal excuse to dispossess them. Their wickedness by itself was not reason enough to give their land to the Israelites unless the Israelites had also proved worthy to take that land. It is also possible that the meaning of the word מורישם is "He destroys them." If that is so, these people certainly had to be wicked as G'd would not decree upon them what He did in Deut. 20,16: "you must not allow a soul of theirs to remain alive."