Why was Avraham adamant that Yitzchak not go there?
Malbim: Hashem took me and my seed from there (like the next verse says). If Yitzchak returns there, it is as if I return there.
Ha'Emek Davar: Yitzchak was considered like a Korban Olah (due to the Akeidah), so he could not go to Chutz la'Aretz. 1
Rashi (26:2) writes that Yitzchak thought to go to Egypt when there was a famine. Had Avraham not told him they he may not leave Eretz Yisrael?! (PF)