It was many different reasons, one reason was that they used it to intimidate the romans. Second is because they weren't as rich as the Romans as the romans were richer they could afford armour and better weapons. With this in mind the Celts also owned less land and weren't fighting to take land, but to simply protect it. Futher reasons are that it is more practical without armour because armour weighs quite alot. They mainly lived in Ireland and Scotland.
battle armor
the Celts wore torques, they also wore amulets to keep away evil spirits and the wrath of the gods and they wore torks.
Celts liked to wear bright clothes and wear jewellery. The jewellery was made from Gold,Silver and bronze When the Celts went to war they would paint themselves with bright blue paint in swirls and patterns and would use a lime mixture to spike up there hair to try to make themselves look scary. women would wear a long dress or skirt with a shawl or a cloak men would wear a tunic with trousers underneath then they would wear a cloak the cloaks where put onto there clothes by patterned metal broaches celts shoes where made out of animal skins
they lived in homes made of hay & straw, the ground was soil & they didnt have anything eletric.
Because Athena was a goddess of war.