If s'lichos and tachanunim are the same thing, as I believe they are, this question is synonymous with "why don't we say tachanun on Tish'a B'Av?" The answer to that, linked here from here is
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.