Do the Romulans really stand out from the crowd? Earth has its share of neat round craters, too: the Borg may have come scooping for cities of interest often enough. Ditto for every Trek player at just about any juncture of their history; who would even notice if an entire culture on Earth went missing in, say, the 1600s?
OTOH, if the Borg value their privacy, then a quick look at the neighborhood would tell them that at this point of history, stealing Romulan cities is preferable to stealing Fed or Klingon ones: the word would be less likely to spread from that tightly controlled and cordoned-off realm.
As for when exactly the Borg hit the Romulans, we can't tell yet. In TNG "The Neutral Zone", they admit to only losing their outposts on the UFP border, not to having zombie cyborgs swarming over major colonies or scooping up cities of the homeworld, but that only tells us the Romulans are not the admitting type. Would be fun to learn that their isolation since the 2310s was due to the Borg. Would be fun to learn something almost but not quite entirely unlike that. Might be we learn nothing about that little detail in PIC, too.
Timo Saloniemi
