Why does the Torah not mention Tzalmonah and Punon in Parshas Chukas, which is where the events described there (at the beginning of chapter 21) took place?
Ramban: The snakes started damaging from when they left Hor ha'Har. Moshe made the copper snake and they took it with them when traveling. The matter ceased only when they came to Ovos.
Chizkuni (42): There the Torah mentions only Ovos, for it is in the southeast corner; when they reached it, they turned to go north.
Hadar Zekenim (21:10): It mentions only Ovos, for there they were stricken with the snakes. It is called Ovos, for there they became Oyvim (enemies) to Hashem.