Every fast day on the Jewish calendar has its set of selichot that are said. The one exception to this is Tisha B'Av. This seems counter-intuitive to me because Tisha B'Av is the day when we mourn the destruction of the first and second Temples, which were destroyed because of our sins. It seems like Tisha B'Av should have selichot even more so than other fast days. So why do we not say them?
No “Tachanun” and no “Avinu Malkeinu,” both typically said on Fast Days, but not on Tisha B’Av, because, although it is the saddest day, and the most severe of the fast days, it contains within it the potential for tremendous joy. It is also called a “Moed,” a special time, as are the joyous holidays Pesach, Shavuot, and Sukkot. This can be, and historically indeed was, a “special time” of punishment but ultimately, it will be a “special time” of Redemption and Rebuilding and Restoration – for the destroyed City of Jerusalem and Holy Temple and the diminished People of Israel, whose spiritual center they’d comprised.
I found this answer. It basically says that since this day of mourning is so great, it is as if the "gates of prayer have been closed".
What everyone else is saying is mostly true, but there is another reason besides for tisha b'av's potential to be a yom tov, and that is because slichos is asking for forgiveness which every other fast day is about, but tisha b'av is only about mourning, not teshuvah. So to sum it all up slichos isn't connected to the happiness of the day, rather the purpose(i.e. Yom Kippur is considered a yom tov yet we still say slichos because the purpose of the day is forgiveness)