Or HaChaim on Numbers 26:3/אור החיים על במדבר כ״ו:ג׳

1 וידבר משה..בערבות מואב, And Moses spoke…in the wilderness of Moav, etc. Why did the Torah have to write that Moses addressed the people "in the wilderness of Moav?" If the location really was of the essence to our understanding what Moses had to say it should have been mentioned when the commandment was introduced, i.e. in verse 1. The reason may be connected to the Midrash we have quoted that Moses had to hand over the people entrusted to him after they had been counted. You will find that whenever counts took place the location is mentioned by the Torah. When we heard about the number the Israelites comprised at the time of the Exodus, the Torah supplied the location, i.e. Ramses. When the Israelites were counted at the beginning of the Book of Numbers we were told that it was in the desert of Sinai. So it may be no more than natural that here too the Torah decided to supply the location for where the count took place. It was here, near the river Jordan opposite the town of Jericho that the loyal shepherd of the Jewish people, Moses, completed his mission. The Torah mentioned that it was after the plague to show that in spite of the plague which had cost so many Jewish lives the total number of Israelites had not shrunk but had considerably increased when we remember that at the first count the whole tribe of Levi had been included and the number given by the Torah was "approx 600.000." This was a testimony to Moses's leadership who could prove he had not lost any of his sheep. This then is the reason why the Torah did not write the location in verse 1. The location was not related to the count but to the fact that Moses used the count to complete his mission and "hand over his flock" to G'd or to a new leader.

2You might argue that the introduction שאו את ראש suggests that G'd did order a census, quite independently of the considerations we have mentioned; the answer is that the words שאו ראש were not intended to demand a census. After all, even if it turned out that the number of the Israelites had shrunk considerably, who would attribute this to their leader? What comparison is there between a shepherd entrusted with sheep and a leader entrusted with people? We would answer that the count now represented the same idea as the one after the golden calf episode. We had been told at the time that when a pack of wolves have attacked a flock, the shepherd counts the remaining sheep to determine the amount of damage he sustained. When many Israelites had died as a result of the golden calf episode, G'd wanted to count how many had survived. In this instance too, many Israelites had died as the result of the debacle at Shittim so that a count to establish how many had remained alive was called for. Moses exploited this opportunity to do his "own thing," i.e. to demonstrate that he had not lost any of "the sheep" under his care. By mentioning the location in connection with what Moses said (verse 3) instead of in connection with what G'd said (verse 1) the Torah was able to make this point. Alternatively, Moses added verse 4 to show that the count was not his own idea but that he carried out G'd's instructions by conducting it.