Ramban: Because the Torah always begins a series of Mishpatim (punishments) with Avodah-Zarah or its worshippers. ¹

Seforno: Having taught the Mitzvah of appointing Batei-Din in every town, the Torah now presents the Dinim of someone who worships Avodah-Zarah and of a Zakein Mamrei, both of whom are not judged in their home towns. The former is judged in the town where he sinned, ² and the latter, in Yerushalayim, at the hand of the Beis-Din ha'Gadol. ³

Sifri: Beis-Din establish what hsppened by means of the two or three witnesses mentioned in Pasuk 6, and the current Pasuk teaches us that whenever the Torah uses the term "Ki Yimatzei", it means through two or three witnesses. ¹

Ramban: To include Chutz la'Aretz, ¹ where the Dinim of Avodah-Zarah are equally applicable. ²

Rashi (in Shoftim, 5:8), Targum Onkelos and Targum Yonasan: It means 'in all your towns'.

Ramban #1: Because that is usually where it takes place.

Ramban #2: Because it is only in Eretz Yisrael that the sinner is put to death by the gate where he sinned; ¹ in Chutz la'Aretz, he is put to death by the gate where he is judged.

Ramban #1: Because, due to the mentality of women, ¹ they are particularly susceptible to being influenced by signs and wonders that are performed before them. ²

Ramban #2 (citing the Sifri): Because, having precluded women from the Din of 'Madi'ach' (rendering a town an Ir ha'Nidachas) - in 13:14, the Torah needs to teach us that here they themselves are subject to Sekilah if they worship Avodah Zarah. ³

Rashi: It is referring to the covenant that Hashem made with Yisrael not to worship Avodah-Zarah.

Seforno: It is referring to 'the covenant that Hashem made with Yisrael over all the Mitzvos. ¹