Left 女 is the indexing woman radical. Many supernatural-creature characters take 女 because folk tales depicted spirits and goblins as bewitching female figures (狐妖 fox-spirit, 妖精 enchantress). The radical pulls 妖 into a family of bewitchment and seduction terms.
Right 夭 supplies the sound yāo exactly. 夭 originally pictured a person bent over, used for 'die young' but also a near-homophone family 妖/腰/邀. The faint resonance of 'young, supple' suits 妖's 'bewitching, enchanting' senses.
