Several culture-specific factors, as others have pointed out: scarcity, attitudes towards ethnic minorities, constructs of beauty, yadda yadda.
I conducted the extremely scientific experiment of searching mavromata “brown-eyed girl” and galanomata “blue-eyed girl” on the Greek lyrics site stixoi.info. Brown-eyed win 87 to 20—despite blue-eyed Greeks being rather rare. And there is a definite cultural construct of brown-eyed girls in Greek song: that of the penetrating glance.
And yet, there’s that lovely poem by Dimitris Lipertis in Cypriot dialect: “Hey, blue-eyes, hark! They’re knocking!/ Oh mother, it’s just the pig.” (about a girl trying to cover up her nighttime tryst).
… OK, it sounds better in Cypriot.