Why were Esau's Canaanite wives a grief of mind to Isaac and Rebekah?
Esau's Canaanite wives caused grief of mind to Isaac and Rebekah because Esau had married Canaanite women. This violated the principle that God was preparing the covenant people of Israel and had to make sure who they married, as illustrated by Abraham's instruction not to take a Canaanite wife for Isaac.
Les explains that Esau married two Canaanite women, and Scripture says they were a grief of mind to Isaac and Rebekah because Esau had married Canaanite women. Les ties this to Abraham's earlier concern to get a bride for Isaac from his own relatives rather than from Canaan, asking why go clear back to a far country when there were all kinds of women in Canaan. The reason given is, "Thou shalt not take a Canaanite for a wife for Isaac." Les says this must be kept in perspective because God was preparing everything for the covenant people of Israel and had to make sure who they married.
Which were a grief of mind unto Isaac and to Rebekah.