Why does it say "va'Ydaber Hashem El Yehoshua"? This expression of strong speech is not used for any Navi other than Moshe!
Makos 11a: Here Divrei Torah were taught. Gra - since the Torah said va'Ydaber, it says so also here. Malbim - here it teaches laws of a murderer that were not taught in the Torah, e.g. he must speak to the elders of the city. No other Navi taught Halachos. 1
Aruch l'Ner (11a): Even Yehoshua may not teach new Halachos. Surely it was a tradition from Sinai, just since it is written in Yehoshua, it is as if it written in the Torah. Some Amora'im hold that one must be able to fulfill verse 4 (he speaks to the elders) as it is written. We do not find so for any other verse of Nevi'im or Kesuvim!
