How could Yitzchak leave Eretz Pelishtim to go and live in Be'er Sheva? HaSh-m told him, "Live in this land" (26:3)!
Ramban (to 23:2), Oznayim la'Torah: Be'er Sheva belongs to Eretz Pelishtim. 1
Now that they did not quarrel with Yitzchak, why did he leave and go to Be'er Sheva?
Rashbam: He was afraid of the Pelishtim.
Malbim #1: The pasture in that area was finished.
Malbim #2: His father had acquired Be'er Sheva and planted an Eshel there.
Ha'amek Davar #1: It was hard for him to live in a small village, or by nature he was a city-dweller.
Ha'amek Davar #2: He wanted to call out in HaSh-m's name (bring others to serve HaSh-m), and this succeeds more in a big city.