It's a source of puzzlement. We mourn 33 days for 24,000 people who died 2000 years ago, but the 6 million who died 60 years ago are lumped together with the millions of others who died in pogroms, crusades and other acts of persecution and remembered on 9 Av.
There are two PC (patently charedi) answers to this question, and neither is satisfactory.
First bogus answer: We don't mourn in Nissan. Really? Tell that to all the unshaven men running around. Fact is we do mourn in Nissan, by observing sefira. Also, we say Yizkor on the 8th day of Pesach (also in Nissan) as well as all the other Jewish holidays. If it's possible to say Yizkor on holidays (holidays that fall in Nissan even) how can it be impossible to hold a remeberance ceremony?
Second bogus answer: Tisha B'Av is the day of mourning for the Jewish people. Really? So why aren't the students of Rabbi Akiva remembered on 9 Av? Why do the 24,000 of them get 33 days, while the other 10s of millions of Jews who were murdered over the years all share the same one day?
The real answer, my friends, is simple. We aren't slighting the Holocaust victims when we commemorate the deaths of Rabbi Akiva's students for 33 days, because the custom of mourning during s'fira originally had nothing to do with them! You'll note, for instance, that the Talmud describes the plaugue which killed them (Yevamot 62b) but says nothing about 33 days of universal mourning. In fact, this practice did not exist for the first 1000 years after their death. A Jew living in the year 600 for example viewed the days between Pesach and Shaviot as a happy time. Not until the Middle Ages, do we encounter Jews observing mourning rituals during sefira.
During the Middle Ages, the days after Pesach were a time of great peril. This was the season for blood libels, pogroms and general military campaigning like the Crusades which sluaghtered the Jews of the Rhineland in May of 1096. As a result, it became ordinary for Jews to be in mourning during the weeks after Pesach, so ordinary, in fact, that the Rabbis of the Middle Ages became convinced that there was somethng inherently harsh about the days between Pesach and Shavuot. The fact that Rabbi Akiva lost 24,000 students during the same period confirmed the judgement, but it was not the impetus for establishing the customs.
[Who agrees with me? Samson Rephael Hirsh in Horeb and Yichiel Michel Epstein in the Aruch HaShulchan among others.]
Sidenote: Though I've argued here that the mourning customs came into being long after the students of Rabbi Akiva had died, it's interesting to note that some say the customs are of earlier origon. May was considered an unlucky month from early Roman times at least and as a result Romans had a custom of not marrying in May.