Radak: They had evil Chukim. You abandoned My good Chukim, and adopted the Chukim of the Emori! I warned you "v'Lo Saki ha'Aretz Eschem b'Tama'achem Osah Ka'asher Ka'ah Es ha'Goy Asher Lifneichem" (Vayikra 18:28).

Malbim: Hashem made two preparations lest Yisrael be contaminated via idolatry and Arayos. (a) He destroyed the Emori, lest Yisrael learn from them to do these abominations ? "k'Ma'ase Eretz Mitzrayim? uch'Ma'ase Eretz Kena'an? Lo Sa'asu" (Vayikra 18:3). The Sifra (131) explains that the Egyptians, especially where Yisrael dwelled, and the Emorim, especially the places that Yisrael captured, were steeped in all these abominations. This is why Hashem took us out of Egypt and destroyed the Emori, lest Yisrael learn from their deeds! Hashem summonsed for a liable generation that Yisrael exact payment from them. How will you do like their deeds?! Also refer to 2:11:1:2, 2:11:2:3.

Radak: Emori was the strongest of the seven Kena'ani nations. This is why only Emori is listed. I destroyed them from their roots ? you did not destroy them with your sword and bow.

Malbim: Do not say that Yisrael destroyed them naturally, for Yisrael were stronger than them. No - "their height was like cedars" in kingship and authority, and "they were like chestnut trees" for strength. Also, "their height" hints to their Sar above. The Ma'arachah and their Mazal was high and succeeding at the time. They had strong fortresses and valiant soldiers. The destruction of them was miraculous.

Radak: It is strong, like "he'Chason li'N'ores" (Yeshayah 1:31).

Rashi: They are oak trees.

Radak citing Targum Yonasan: They are chestnut trees.

Rashi #1: These are the officers above (in Shamayim) and the lower officers, respectively.

Rashi #2: A hornet blinded the Emorim above and castrated them below.

Malbim: The Peros and roots were wiped out ? no memory of them remained.