Because you have to make trade-offs. Because you have to design an electoral system that optimizes a number of goods at once, not just one.
Look, if your only concern is that we ought to have a balance between men and women in Parliament, there's an easy solution: pass a law that says you will refund election expenses for only—whatever the number is—169 men and 169 women, and in the next election you will have an equal number of male and female candidates.