It’s the same reason Beis Din punishes, for the sake of the rest of us. To show that Yesh Din Veyesh Dayan. It is hard to grasp why some get something here and some don’t, but it is all about establishing the Malchus of HKBH on this world.
Titus got something, a token of recognition that Hashem is here and is aware and is able to punish. It was not his punishment for his crimes. Carpet-chewer got away with it, although his country did suffer. Obviously, that was deemed sufficient to show how ultimately light will prevail over darkness.