Rashi: To teach us that even his bones would not cross the Yarden (to be buried in Eretz Yisrael).

Oznayim la'Torah (based on R. Bachye): Refer to 4:21:2:2. Because of the two reasons that Moshe's bones were buried in Chutz la'Aretz: 1. In honor of - and in order to merit - the Dor ha'Midbar, who did not enter Eretz Cana'an and who were buried in the desert; 2. In honor of - and in order to merit - those who died in Chutz la'Aretz in subsequent exiles.

Ramban #1 (on Pasuk 21) He was encouraging them to learn from him now, since he was destined to die there in the land of Mo'av, and would not be available to teach them when they entered Eretz Yisrael. ¹

Ramban #2 (on Pasuk 21, according to Kabalah): It was a continuation of the warning (Pesukim 16-19) against making any images ² - inasmuch as Hashem denied him entry into Eretz Yisrael for fear that they would worship him ³ , a clear indication as to the seriousness of adopting an image as a deity. ⁴

Seforno: Because, since he would not cross the Yarden with them, they were susceptible to corrupt their ways, and that consequently, it was urgent that he warn them now. ⁵