From Wikipedia:
"Ivins was a Roman Catholic. The Frederick News-Post has made public several letters to the editor written by Ivins dealing with his religious views.[32] These were cited in the Department of Justice summary of the case against Ivins as suggesting that he may have harbored a grudge against pro-choice Catholic senators Daschle and Leahy, recipients of anthrax mailings.[33] In a letter expressing his belief that Jews were God's chosen people, Ivins stated, "By blood and faith, Jews are God's chosen, and have no need for 'dialogue' with any gentile."[34] Ivins praised a rabbi for refusing a dialogue with a Muslim cleric.[34]"
Also they were 2 of the most important senators at the time one was majority leader the other chairman of the Judiciary Committee.
Or perhaps it was because they stood in the way of the Joooz plans for world domination.