You're talking about the Spartans. They were one city state in a sea of republican cities. And the rest of Greece fucking hated them. They were like an entirely different breed of people, who rarely accepted outsiders.

The only reason we even know anything about their customs is because of a single guy from Athens who happened to be friends with one of the kings

Anyway, the reason for those abortions was because every single Spartan citizen was considered a warrior, they were basically all assigned land and slaves by the government. It was probably a measure to ensure no shit genes since only Spartans were allowed to fight.