Ramban: Refer to 1:4:1:1 & 2. The Torah is informing us that Moshe also began to explain the Torah only after he had captured the lands of Sichon and Og - and distributed it among the two and a half tribes.

Seforno: After Moshe captured inhabited land, where they were able to practice the Mitzvos without fear of being attacked, he proceeded to expound on the Torah and to warn them to observe it.

Oznayim la'Torah: Because, upset at not being able to distribute Eretz Yisrael to the tribes, and further aggravated by the taunts of Dasan and Aviram when they declared "ve'Lo Nasata lanu Nachalas Sadeh va'Karem", it was a great thrill for him to distribute Eiever ha'Yarden to the two and a half tribes - particularly to Reuven, the tribe of Dasan and Aviram. Consequently, the Torah mentions it at every opportunity, to give Moshe th feeling of satisfaction.