Style guides generally update to represent the editorial team's preferences, to reflect changes in the wider world (e.g. see the comment in this thread about "Constaninople"→"Istanbul", or to make a point (see the New Yorker's continued use of diaresis in coöperation etc.[1])

[1]: https://en.wikipedia.org/wiki/Diaeresis_%28diacritic%29#Engl...