Maimonides defines the obligation to fast as an extension of the obligation to call out or even scream out to G-d in times of trouble. Just as a person might scream and protest or start a hunger strike in front of an embassy to protest its policies, so do we call out in protesting prayer and we go on a hunger strike on a fast day. The fasting sends a message. It is a form of prayer and protest. 

On most fast days, we deepen the message of the prayerful fast by praying more. We recite Selichot, supplications in which we literally scream out to G-d. “Answer us! Hear our voices!” These are the cries of a fast day, including Yom Kippur. Not so on Tisha B’Av. On this fast day, we cut out our prayers of supplication, even our routine supplications known as Tachanun. We do not say Selichot and confession. What do we do? We lament. We cry with the book of Lamentations. We weep with the Kinnot, the lamentations written by Jews in the Middle Ages who had their own “tzaros.” 

The idea of a fast is to call out to G-d. Usually the fast does so in the form of calling and shouting out to G-d in prayer. On Tisha B’Av, the same idea is conveyed through wailing. The word “oy” is a refrain of the holiday. But why do we not have more supplications on Tisha B’Av? The answer is that it says in the book of Lamentations, Eichah, that “even as I pray and call out,  my prayers are cut off or closed up.” The destruction of the Holy Temple made us feel that our prayers had not been answered, that a wall of iron had gone up between us. We shy away from normative prayers on Tisha B’Av to convey our desperation. Maybe when G-d sees how bereft of hope we feel, he will have mercy on us and restore our hope for salvation.