Avraham said to him, "Be insistent not to take my son back there. G‑d, the G‑d of the heavens— who took me from my father's house [in Charan] and from the land [of Ur Kasdim] where I was born; who spoke to me [about my needs], and swore to me [at the Covenant of the Parts], saying, 'I will give this land to your descendants'—He will send His angel ahead of you, and you will take a wife for my son from there.
-- Breshis 24:6-7
Why was Yitzchak not allowed to leave Cana'an? (v. 6)
Minchah Belulah: Yitzchak had the status of a 'perfect burnt-offering' and was not permitted to leave the Land of Israel.
Kli Yakar: Avraham did not want Yitzchak to marry a girl from Cana'an (v. 3) for the local people had a genetic disposition to self-indulgent desires. The family of Lavan and Besu'el did not possess this genetic predisposition but, on the other hand, they were idol-worshippers.
A tendency to idol worship, however, is not a genetically inherited quality, but rather a cultural phenomenon. Therefore, Avraham requested that: a.) Yitzchak's wife should not be from Cana'an but from "my birthplace," so that she would be of good genetic disposition. b.) He insisted "not to take my son back there," i.e., the girl must be removed from the idol worshipping culture and brought to Yitzchak, and not the other way around.
The Rebbe's Teachings
Avraham's Request (v. 6-7)
Minchah Belulah writes that Avraham did not want Yitzchak to leave the Land of Israel because he had the status of a "perfect burnt offering." However, this explanation does not appear to be consistent with Rashi's explanation later, in Parshas Toldos:
On the verse, "G‑d appeared to him and said, 'Do not go down to Egypt! ... Settle in this land...'" (26:1-3), Rashi comments, "[Yitzchak] had in mind to go down to Egypt as his father had gone down in the days of the famine. G‑d said to him, "Do not go down to Egypt! You are a perfect burnt offering, and being outside the Land [of Israel] is not fitting for you!"
(Yitzchak was consecrated to G‑d as a burnt offering on the Altar [during the incident of the Akeida—see above 22:2ff.], and he was not permitted to leave the Land of Israel, just as a burnt offering may not be taken out of the Temple Courtyard—Mizrachi.)
This begs the question: Yitzchak clearly knew that he had been placed onto the Altar as a burnt offering at the Akeida, so how could he have ignored the prohibition of leaving the Land?
It seems therefore that until G‑d told Yitzchak, "Do not go down to Egypt!" it was not yet prohibited for him to leave the Land of Israel. And that is why Yitzchak innocently planned on leaving the Land, until he was told otherwise by G‑d.
Thus, in our parshah, when Avraham told Eliezer, "Be insistent not to take my son back there" (v. 6), he could not have possibly meant that Yitzchak should not leave the Land of Israel because he is a "perfect burnt-offering" (as Minchah Belulah writes), since Yitzchak had not yet been prohibited by G‑d from leaving the land.
This, however, leaves us with the question: Why did Avraham not allow Yitzchak to leave the land of Cana'an? And if the land of Cana'an was so special, why was he not allowed to marry a girl from there? (c.f. Kli Yakar)
Rashi appears to remain silent on this issue. However, since it is a question which needs to be explained at the literal level, Rashi must explain the matter somewhere, in keeping with his goal to clarify every issue that is not self-understood at the literal level.
It could be argued that Rashi clarified this matter in his comment on verse 7: "Avraham said to him, 'Now He is the G‑d of the heaven and the G‑d of the earth, because I have made it habitual for creatures to mention Him, etc.'"
At first glance, Rashi's comment is somewhat perplexing. What forced Rashi to conclude that Avraham told Eliezer ("Avraham said to him") this lengthy explanation about publicizing G‑d's Name? How was this connected with Eliezer's mission to find a wife for Yitzchak?
Rashi's comment here was prompted by our previous question: Why did Avraham demand that Eliezer bring a wife from his birthplace back to Cana'an? Since this point is crucial to our understanding of the story, Rashi concluded that Avraham must have given some sort of explanation to Eliezer.
In order to clarify exactly what Avraham's explanation was, let us examine Rashi's precise choice of words:
"Now... I have made it habitual (שהרגלתיו) for creatures to mention Him." The use of the word "habitual" suggests that Avraham merely brought the people of Cana'an to a superficial awareness of G‑d. They merely "mentioned Him," habitually.
This explains why Avraham said not to "take a wife for my son from the daughters of the Cana'anites, amongst whom I live" (v. 3): because their recognition of G‑d—despite Avraham's efforts—was only superficial and habitual. Avraham thus preferred to take a wife from his own family, who were naturally better and more refined people, closer to the qualities of Avraham.
Nevertheless, Avraham did not want Yitzchak to leave Cana'an and return to Charan, because "when He took me from my father's house He was the G‑d of the heavens but not the G‑d of the earth, because mankind did not acknowledge Him, and His Name was not commonplace on the earth." I.e., in Charan, Avraham had not succeeded in making G‑d's Name known (even habitually or superficially). Therefore, it was preferable for Yitzchak to stay in Cana'an, where at least it was "habitual for creatures to mention Him."
(Based on Likutei Sichos vol. 15, p. 155ff.)