i don't know what the in-canon story is, but the behind the scenes story is that when they switched to prosthetic foreheads for the romulans it was easier to get guest actors to portray romulans b/c it meant that the actors wouldn't have to shave any parts of their eyebrows.
seems rather superficial, but that's the scuttlebutt.
scuttlebutt's wrong. they added the ridges to make them 'mean' looking next to ridged Klingons. IIRC, Westmore did it.