until lag b'omer we mourn the 24k students of Rabbi Akiva who were wiped out in a plague (Yevamot 62b). Why do we mourn this and not many other tragic events in our history (ex.pinchas affair, korach, quail, the various tragic wars in tanach)? also why is this mourned longer than any other event?
The Gemarah which you quoted above (Yevamos 62b) says after Rabbi Akiva's 24,000 disciples died, the world was desolate until Rabbi Akiva came & taught five new students. Rashi there says the reason for this was because of the Torah which was forgotten by their passing. The Torah is the lifeblood of the Jewish people. It is also the reason the world is continuously in existence.
The Torah, at the end of creation (Bereishis Perek 1 Pasuk 31) says יום השישי . Rashi, apparently bothered by the extra "Hay" in the word Shishi brings a Gemarah Shabbos 88a which says the entire six days of creation was waiting for the 6th day of Sivan - the day when thousands of years later the Jews would accept the Torah. Without the Torah the world cannot survive.
It is quoted from the Chofetz Chaim (Died 1933), although I don't have a source for you to look it up, that one of the reasons why G-d made different time zones in the world was so that there can always be people studying Torah throughout the day & night. Otherwise the world would cease to exist.
This is the reason why we specifically mourn the students of Rabbi Akiva passing away, as a reminder for us how important our learning is for ourselves & the world at large.
The above is my own theory, I did not see this quoted somewhere else. I welcome comments on it.