I think, the question should be
Why is Trichloroacetic acid more acidic than Tribromo acetic acid ?
Threre exists 3α- H atoms in the acetic acid molecule.
We can get three halo substituted acetic acid by replacing these H atoms by 1 or 2 or 3 halogen atoms. Halogens are more electrongative than carbon . So the halogen atoms linked with the α- C atom render -I effect in the molecule by attracting bonded electron pairs towards them and the effect is transmitted to O-H bond in carboxylic group and this phenomenon facilitates the release of H+ ion in its aqueous solution. This -I effect is enhanced by the number of Halogen atoms introduced as well as electronegativity of the halogen atom introduced. As the electronegativity decreases down the group Br is less elwctronegative than Chlorine, Hence in Trichloroacetic acid -I effect is stronger than that in tribromoacetic acid. So Trichloroacetic acid behaves as stronger acid than Tribromoacetic acid