Malbim: The plague already began to strike them, and Hashem returned His hand from swallowing (eradicating them). Also this was for His honor; it says so three times in this Parashah. He did so (a) in Egypt, before Yisrael received the Torah, (b) in the Midbar, at the beginning - they accepted the Torah, but did not yet purify from the idolatry of Egypt, and (c) the generation of the children after 40 years, before they entered Eretz Yisrael, which is prepared for Kedushah. As long as Hashem did not bring them into the land, there would be Chilul Hashem if He did not fulfill His word and Shevu'ah to the Avos.