Why does it say "Zeh ha'Davar"?
Rashi: He circumcised them via Dibur. Do you think that you will inherit the land [while you are] Arelim?! Hashem told Avraham unlike this
Malbim: Others refrained because the north wind did not blow, or because they did not know when Machaneh Yisrael will travel. Tzadikim were not concerned for these. Nowadays that many are not concerned for the north wind, "Shomer Pesa'im Hashem" (Tehilim 116:6). If the Machaneh would travel, they would stay with the baby until he healed.
How can the verse say that everyone was Arel? A Midrash says that Shevet Levi circumcised, and those who were less than 20 at the time of Chet ha'Meraglim were alive, and they circumcised when they left Egypt!
Refer to 5:4:1:3.
Why does it say that they died in the Midbar "ba'Derech b'Tzeischem mi'Mitzrayim"?
Malbim: They died because they had not purified themselves from the Tum'ah of Egypt and their awful mindset.