They didn't bother explaining why the Romulans had ridges in TNG but not in TOS.
They kind of did. Spock passes as a Romulan without suspicion in "Unification", thus both forehead styles coexist.
Mintakans were proto-Vulcans. With ridges.
Flat-foreheaded Vulcans and Romulans were perhaps just "more evolved".
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