What was so bad about Esav’s wives?

In Parhsat Toldot, Breisheet 26:34-35 we read about Esav’s wives:

Eisav was 40 years old when he married Yehudit, the daughter of Be’eri, the Hiti, and Bosmat, the daughter of Eilon, the Hiti. They were a source of “morat ruach” to Yitzchak and to Rivka.

Sfrorno explains what “morat ruach” means:

They became like a razor and a knife, shortening the lives of both Yitzchak and Rivka. The expression “morat” is the same as in Shoftim 13:5 where the angel told the wife of Manoach that the as yet unborn Shimshon must be a Nazir and “u’morah lo ya’aleh al rosho”, no “hair-trimming tool must touch his head.”

Even now Yitzchak did not realize the extent of his son Esav’s wickedness. He did not protest his behavior with a single word. This led to his ultimate error in setting out to bestow the blessing on his son Esav. This, in turn led to Esav getting the blessing or advice that he would live by his sword. Because of this blessing he hated his brother to the extent that Yaakov had to flee for his very life to another country in order to get away from his murderous intentions.

Chizkuni comments:

Both of these daughters in law became sources of frustration to both Yitzchak and Rivkah. We see this from Rivka’s justifying her wishing to take a wife for Yaakov from her family (27:46) by telling Yitzchak that if Yaakov were to take wife from the local Hiti population she would get fed up with her life. What can describe more graphically her and her husband’s frustration at the choice of wives Esau had made?

According to Bereshit Rabbah 65:4, Yitzchak was the first to experience dissatisfaction with Esav’s wives’ conduct. It is explained that the reason is psychological. Rivka, who had grown up in an environment of pagans, did not become affronted so easily by Hiti women not yet having abandoned every nuance of their former pagan backgrounds, whereas Yitzchak was used to being surrounded only by holiness and was therefore far more sensitive to this.

Rashi explains that Esav’s wives caused resentment or bitterness (as in Dvarim 9:24, “you were rebellious”). All their deeds were to anger and cause grief to Yitzchak and to Rivka because they worshipped idols.

In the next verse, Breisheet 27:1, Rashi comments that Yitzchak’s eyesight faded because of the smoke of these women. The source is from Midrash Tanchuma 8 which says that the smoke was raised by them when burning incense for idol worship.

We see that the fact that Esav married idol worshippers was a real problem for Yitzchak and Rivka and they didn’t want Yaakov to end up in the same boat.