Question
Why do only male first born have to be redeemed?
Answer
The sanctification of the first born (which is why they require redemption) can be regarded as a consequence of the Tenth Plague in Egypt, which killed all the Egyptian male first born, human and animal; see Exodus 13:11-15. Since only males died in Egypt, only males need to be redeemed. Other laws that commemorate the Exodus, such as the laws of the Passover Seder, apply equally to men and women.