I heard two other answers that no one has yet mentioned to answer the OP:
1) The Ramban says that the days of Sefiras ha’Omer are like a “Chol ha’Mo’ed” between Pesach and Shavuos. The truth is, therefore, that ALL of them should be celebrated. However, we mourn the deaths of the students on all of the other days of Sefira. When this is not applicable, we celebrate that day itself! (Heard from a Chashuva Rav)
2) Rebbi Akiva was also supposed to die during this time period. We celebrate that he was spared. (Heard from a very fine Yid, but not the same level as the first.)