Because they trained boys AND girls to fight from a young age, had mandatory military requirements, got rid of the weak and useless and were very strict and harsh in both mitary life and society. Their society had very very strict rules and laws that were enforced brutally. Discipline was harsh and fast. There was no such thing as 'human rights'. It was an excellent military society but not a happy one. Theres a reason the term 'spartan' is used as a descriptive for a frugal, dull, austere unhappy situation. ie. the room was very spartan in its decor. The prison meeting room was very spartan. He was very poor and lived quite spartan life. The Amish are quite spartan in how they choose to live.
The Spartans were ruthless in their training of their young. If a baby was born that did not meet the standards it's head was bashed against the rocks and thrown into the ocean. They trained their boys from nearly the time they started to walk for military service.
because spartans focused on their military they train young ages. but hey guess what when cyprus the great tried to invade them they lose and guess what who helped....it's athens,, when athens was under attack of cyprus the great tgey seek help from spartans but they refuses, bcos athens are smart the drive them away and they helped sparta becos if sparta lose cyprus the great could defeat them