Basically, we have three options for these kinds of words: “cooperate,” “co-operate,” and “coöperate.” Back when the magazine was just getting started, someone decided that the first misread and the second was ridiculous, and adopted the diaeresis as the most elegant solution with the broadest application. The diaeresis is the single thing that readers of the letter-writing variety complain about most.
We do change our style from time to time. My predecessor (and the former keeper of the comma shaker) told me that she used to pester the style editor, Hobie Weekes, who had been at the magazine since 1928, to get rid of the diaeresis. She found it fussy. She said that once, in the elevator, he told her he was on the verge of changing that style and would be sending out a memo soon. And then he died.
This was in 1978. No one has had the nerve to raise the subject since.