Kicking to the lower body vs. the head:
1. is much faster
2. is much harder to block
3. is stronger
4. compromises your balance much less
5. allows you to be in a better defensive posture while delivering the technique
You may think that kicking someone in the head is great because it can be a KO, but many kicks such as a Muay Thai style to the thigh will leave your opponent a sitting duck with severely diminished balance.