Seems to be an older convention in linguistics. Romanizations of Japanese also switched from circumflexes (Tôkyô) to macrons (Tōkyō) at some point in time fairl

Seems to be an older convention in linguistics. Romanizations of Japanese also switched from circumflexes (Tôkyô) to macrons (Tōkyō) at some point in time fairly long ago—I think the English-language Japanese journal I saw using that convention systematically was from the late 1950s, and its recent issues definitely don’t use it. Perhaps a circumflex was easier to typeset, like with logicians switching from Ā to ¬A and the Chomskyan school in linguistics switching from X-bar and X-double-bar to X' and XP?