I believe people generally find blue or green eyes to be attractive since they’re less common than brown eyes. Colored eyes also seem to be an even more prominently desirable trait in countries where a vast majority of the population has brown eyes: the rarity of blue/green eyes makes them appealing.
I actually think brown eyes are lovely for the same reason, but in reverse. I live in a household of seven, and my siblings, parents, and grandmother all have blue eyes (I do too). Growing up, almost every single one of my closest friends had blue or green eyes, and that’s true of my friends today. The eyes I’ve looked into most frequently have been colored. Brown eyes are more rare within my circle of friends, family, and acquaintances, so I think they’re fascinating.