If I had to take a stab at it I would say that breasts are attractive to people due to breast feeding and mother/child bond (even babies that aren't breastfed still have the biological conditioning to want to breastfeed). That's why on average most people find them attractive in some way and why even women can find other women aesthetically beautiful even if they are straight.
However, I also think part of it is cultural. If our culture for example allowed men to dress and run around with their genitals exposed and people were used to seeing them every day maybe people would also develop neutral standards of what makes a beautiful or less beautiful penis and hence attractive men. We have standards of what makes beautiful breasts because we see a lot of them. We have no standards of what makes a beautiful vagina because we don't see them as often.
However, I also think part of it is cultural. If our culture for example allowed men to dress and run around with their genitals exposed and people were used to seeing them every day maybe people would also develop neutral standards of what makes a beautiful or less beautiful penis and hence attractive men. We have standards of what makes beautiful breasts because we see a lot of them. We have no standards of what makes a beautiful vagina because we don't see them as often.