No new "official" Jewish commemorative days?

Question

The last "official" Jewish commemorative day was added 2000 years ago. Why can't there be any new ones (approved by ALL), considering all that's happened to Jews since?

Jewish communities have local commemorations. In Egypt we had Purim Mitzrayim, to commemorate a narrow escape in 1524. But it is not the case that the only "official" commemorations that apply to all Jews are those commemorating times when all Jews lived together in Eretz. At the time of the fall of the Second Temple (last "official" Jewish commemoration), there were large communities in Alexandria and elsewhere in the Roman Empire, and in Babylonia.

Answer 1

Since you asked for a day "accepted by all", there should be an authority that is accepted Halachicly for all.

Since the times of the Bavli Gemmorah, there's no such authority. No single Rabbi is universally accepted by all and in all rulings. The closer we get to the current days, the lesser accepted (and more localized) the Rabbis become. Some proposed that the Israeli State (and its Rabbonut) would be such an Authority, but turned out it is Halachicly completely unacceptable abroad and by the local Haredi community.

So, till the coming of the Messiah (who's gonna cancel the existing days anyway), we're stuck with what we have.

Answer 2

Partial answer regarding why you shouldn't expect to find agreement on the creation of a new day to commemorate a tragedy.

R. Dr. JJ Schacter in this Tradition article describes some of the opposition to establishing a day mourning the (at the time ongoing) Shoah as follows (emphasis mine):

R. Herzog inquired whether R. [Yitzhak Ze'ev] Soloveichik would support the establishment of a “day of mourning (yom evel)” for the tragedy and destruction that was occurring in Europe... R. Soloveichik opposed R. Herzog’s initiative, basing his objection on a passage in one of the kinot recited on Tish’a be-Av dealing with the Crusades. The author of this kina, entitled “Mi yiten roshi mayim,” describes the devastation of the German communities of Speyer, Worms, and Mainz caused by the Crusaders at the end of the eleventh century. The kina is explicitly historical in orientation and notes the specific dates when the destruction of these Jewish communities took place: Speyer on the eighth of Iyyar, Worms on the twenty-third of that month and again on the first of Sivan, and Mainz two days later. Its author then addresses the question of why a kina about the destruction wrought by the Crusades is recited on Tish’a be-Av if, as he just explicitly indicated, it did not occur on—or even within two months of—that day. His answer is clear and instructive: “Since one may not add a time [to commemorate] destruction and conflagration . . . therefore today [i.e., Tish’a be-Av] I will raise my cries of woe (Ve-khi ein le-hosif mo’ed shever ve-tav’era . . . tahat ken ha-yom livyati a’oreira).” One cannot institute new days of commemoration for tragedies that followed the destructions of the Temples. There is one day in the Jewish calendar designated to commemorate Jewish tragedy and that day is Tish’a be-Av. Mourning for—and commemoration of—all subsequent Jewish tragedies are to take place on that date and on that date alone, wrote the author of this kina. As a result, argued R. Soloveichik, no special day of commemoration was established for the destruction wrought by the Crusaders; the appropriate day on which to remember this tragedy is Tish’a be-Av. He applied the principle deliniated in this kina in the context of the Crusades to the situation he was facing in 1942-1943. On the basis of this statement, he concluded that establishing a special day of mourning for the growing number of Jews being exterminated in Europe would be inappropriate and contrary to Jewish tradition.