משך חכמה — על כן שם העיר באר שבע עד היום הזה

Q — Why does the Torah credit Yitzchak's oath with naming Be'er Sheva "עד היום הזה" when the city had already been named after Avraham's earlier oath with Avimelech?

A — Avimelech violated the first oath by expelling Yitzchak — so the שבועה lapsed and the name with it; only the second oath, between Yitzchak and Avimelech, was kept, and it is THAT oath that established the name Be'er Sheva permanently "עד היום הזה."