Because they were Ishmaelites A mixture of people all called by one general name, Ishmaelites or Arabians, who used to wear ear-rings; but the greatest and the ruling part of them were Midianites.
Related Commentaries of Judges 8:24
Judges 8:24
24 And Gideon said unto them, I would desire a request of you, that ye would give me every man the earrings of his prey. (For they had golden earrings, because they were Ishmaelites.)