Rashi: The Torah views eating from their sacrifices akin to participating in their idol-worship, because one thing leads to another, and the next step is taking his daughter as a wife for one's son.

Ramban: Straying after the gods of the Cana'anim, eating from their sacrifices ¹ and taking their daughters for our sons are all prohibitions that stem from making a covenant with them.

Seforno: Because entering into a covenant with them sometimes leads initially to participating in their festivities and from there to joining them in their idolatrous practices, and sometimes to licentiousness. ²

Avodah Zarah, 18a: To implicate the sinner already from the time he accepts the invitation to participate. ¹