Because They Did Not Respect Each Other
Yebamot Tractate, from the book Ein Yaakov
It was said that Rabbi Akiva had twelve thousand pairs of disciples, from Gabbatha to Antipatris and all of them died at the same time because they did not treat each other with respect. The world remained desolate until Rabbi Akiva came to our Rabbis in the South and taught the Torah to them. These were Rabbi Meir, Rabbi Judah, Rabbi Yossi, Rabbi. Shimon and Rabbi. Eleazar ben Shammua; and it was they who revived the Torah at that time. A teacher taught: All of them died between Passover and Pentecost. Rabbi. Hama ben Abba or, it might have been Rabbi. Hiyya ben Abin said: All of them died a cruel death. What was it? — Rabbi Nahman replied: diphtheria.