Ramban: Because it was situated by the crossings of the Yarden, and it is is from point that they actually crossed into Eretz Cana'an. 1
Why did Hashem see fit to add the words "Ka'asher Meis Aharon Achicha"?
Rashi #1: From here we see that Moshe coveted the same death as Aharon. 1
Rashi #2: It is to inform him that, since it was decreed that they could both not enter Eretz Yisrael because they had sinned by failing to sanctify Hashem's Name 2 and Aharon had died in the desert, he was no better than him, and he must therefore die too.
Why, whenever the Torah mentions Moshe and Aharon's death, does it always see fit to mention the reason why they died.
Rashi: Because Moshe requested it, so that people should not think that they did not enter Eretz Yisrael due to a lack of Emunah, like the rest of the generation that left Egypt. 1 The Torah therefore fulfilled his request, to let people know that this was their only sin. 2
Rashi: It can be compared to two women who received Malkos in Beis-Din, one for committing adultery (See Sifsei Chachamim), the other, for eating unripe figs of Sh'mitah (See Sifsei Chachamim). The latter requested of Beis-Din to to announce her sin, so that people should not accuse her of the more severe sin of adultery.