http://revach.net/article.php?id=1863

Section: Taryag Mitzvos   Category: Minchas Chinuch
Minchas Chinuch: Why Was the Shemen HaMishcha Discontinued?

The Rambam says that the only Shemen HaMishcha ever made was the one that was made by Moshe Rabbeinu. The Shemen HaMishcha that was made by Moshe Rabbeinu was hidden by Yoshiyahu HaMelech toward the end of the first Bais HaMikdash and it remains hidden until today. None of the Kohanim Gedolim from that time on were anointed with the Shemen HaMishcha.

The Minchas Chinuch asks that since the Mitzvah of making the Shemen HaMishcha is a Mitzvah L'Doros (for all generations) and not a Mitzvah L'Shaah (for a specific time) why wasn't new Shemen HaMishcha made during the time of the second Bais HaMikdash for the purpose of anointing the Kohanim Gedolim?

They Minchas Chinuch answers that the Mitzvah of anointing the Shemen HaMishcha on the Kings and Kohanim Gedolim is a Mitzvah L'Doros, however the Mitzvah to make the Shemen HaMishcha applies only to Moshe Rabbeinu not to the rest of Klal Yisrael. Hashem Yisborach commanded Moshe Rabbeinu to make Shemen HaMishcha that will last forever. Therefore once the Shemen HaMishcha was hidden Moshe Rabbeinu was not around to make new Shemen HaMishcha.