The feeling is that tisha b’Av includes all tragedies, including the shoah.
In fact, there’s a piyut that my shul says in the morning written by a rebbe referring to the shoah along with all the older ones.
Some feel that we would then have to have a day set aside for all tragedies and not just pick the shoah: a day remembering tach v’tat, a day for the Spanish Inquisition, etc.