oomis – I think that you are missing the point of those discussing Tisha B’Av as our national day of mourning. The point is not that we don’t remember the Holocaust, but rather the day to mourn it is on Tisha B’Av itself. The point is that the Holocaust is the most recent manifestation of the fact that we are in Galus, and so on Tisha B’av it is part of the tragedy. As you mentioned, since the Shoah is somewhat more personal to us, by thinking about and discussing the Shoah on Tisha B’av, and then realizing that this, too is a result of the Churban, it will help us achieve that deep sense of mourning that is so incumbent on us to feel. We were always encouraged to read Holocaust biographies during the three weeks and on Tisha B’Av. Additionally, I remember once hearing a shiur discussing that Tisha B’Av is also an appropriate time to mourn personal tragedies in our families and communities, which are also a result of the lack of Hashraas Hashichina in our lives due to galus.
I am not sure about your historical generalizations however. I believe the number of Jews killed during the period surrounding the Churban Bais Sheni was in the multiple millions as well. And in terms of the “systematic and organized” murder of Jews, the Spanish Inquisition was pretty systematic and organized, as were the pogroms during Tach V’Tat (although obviously the actual numbers killed do not compare). I am not trying in any way to Chas V’Shalom minimize the Holocaust. Simply to point out that this has been a long and difficult galus, and the point of Tisha B’Av is to mourn all the aspects of it.
In terms of Yom Hashoah itself, I believe a previous poster has mentioned that the complete “official” name of the day is “Yom HaShoah V’Hagevurah”, and it was picked because it was the day associated with the Warsaw Ghetto uprising. Many of the early secular Zionists were embarrassed by the Holocaust and felt that the Jews went “like sheep to the slaughter”. The uprising was something they could take pride in, as it was the type of “gevurah” they approved off. (Tangentially there certainly was a tremendous amount of spiritual gevurah under the harshest circumstances throughout the Holocaust. . .) So even if one would feel that for some reason the Holocaust needed a separate day of commemoration, there are actual problems with the specific day, from a Torah hashkafa.