Chabad.org — English translation + Rashi
Torah Temimah (Menachos 28b): the chatzotzros that Moshe made were valid for him but not for later generations — “לך ולא לדורות.”
English: Those that Moshe made were valid for him and valid for generations. The chatzotzros were valid for him and invalid for generations. What is the reason for the chatzotzros? … Because the verse says “make for yourself” — for you and not for generations.
Generational limit — the trumpets of the wilderness were unique to Moshe’s era.
Later generations must make their own; Moshe’s pair does not transfer.