We know, That the Meraglim came back on the night of Tisha BeAv, and Hashem said, you cried for nothing on this night, I promise you will be crying for generations on this night. Seemingly, it was established a day of sadness, because of the sin of the Meraglim. It seems like, the same is true with Shivah Asar BeTamuz. Since the Luchos were broken on that day, so it became a day of Puranios. So why do we only say this regarding Tisha BeAv?

ב"ה Shalom Indeed, as you've said the 17th of Tammuz also had its many tragedies as the Mishna in Ta'anit 4:6 enumerates. The Korban Tamid was cancelled, the walls of Yerushalayim were breached, Afostomos burnt the torah, and an idle was erected in the Bet Hamikdash. So our Rabbis didn't underestimate the tragedies of Tammuz. Neither did they overlook the tragedies of Tevet, in which on 3 consecutive days tragedies befell the Jewish people. However, Tisha B'av is the climax of the Churban, and Chazal wanted to teach a very deep lesson in regard to Tisha B'av. The wailing of the Mergalim wasn't just a wailing and wining from being spoiled, it was an expression of the despising Eretz Yisrael .It was a sign of being ungrateful in the fullest extent to Hashem , and a sign of rejection of our destiny as a people. Rav Meir Simcha of Dvinsk says that Hashem frowned upon the sin of the Meraglim more than he frowned upon creating of the "Agel" = Golden Calf . In regard to the Golden Calf, the Torah says that Hashem renounced the punishment he planned. See Shemot 32:14). But in regard to the sin of the Meraglim, Hashem says "do not ascend…for Hashem is not in your midst. " The Ramban to Bamidbar 14: 18, says that in the sin of the meraglim Hashem did not mention the full 13 attrbutes of mercy. The attribute that Hashem extends His kindness to a thousand generations, was not mentioned when Moshe prayed after the sin of the Meraglim. That was because Moshe did want to invoke the merit of Avot, because the merit of Avraham ,Yyitchak and Yaakov, would not help them, if they despised the gift Hashem gave the Avot. In short, we see the sin of the Meraglim had ramifications beyond what most people think. Therefore, on the eve of Tisha b'av when the Meraglim cried not to go to into Eretz Yisrael, we have been crying for generations for the destruction of the Bet Hamikdash. In the time of the Second Beit Hamikdash, not all Jews had then retuned to Eretz Yisrael from Bavel. :The Gemara in Yoma 9b , tells us of how Resh Lakish had scolded the Jews who remained in Bavel. " Had you rendered yourselves a solid bloc like a wall and all ascended to Eretz Yisrael in the days of Ezra, you would have been likened to silver, which does not rot , in the sense that you would have merited experiencing the Divine Presence in all its glory. Now that you ascended like doors, and only some of you came to Eretz Yisrael, you are likened to cedar, which rot infests, and you merit experiencing only partial revelation of the Divine Presence." This is the explanation given to explain Resh Lakish's behavior not taking the hand of Rabba Bar Bar Channa and saying to him that "Hashem hates you." We have to do Teshuva for the sin of the Meraglim, perhaps as Rav Yehuda Halevi wrote in his book the Kuzari, that Yerushalyim shall be rebuilt when the Jewish people see Yerushlayim as the epitome of all their desires. All the best