Ramban #1: Because Hashem destroyed every last Egyptian horse, chariot and rider, ¹ leaving Egypt a lowly nation, without horses, without chariots and without an army.

Ramban #2 (citing the Ibn Ezra): Because all those who chased after Yisrael were destroyed, together with their offspring in Egypt, ² so that nothing remained of the entire family. ³

Seforno: Because all the Egyptian officers together with all the soldiers perished at the Yam-Suf, a blow from which they had not recovered forty years later, when they had still not been replaced.