In short we developed larger brains than our ancestors (which meant larger heads for infants) at the same time that we also developed an upright walking posture (which narrows the birth canal). Additionally, we also have some adaptations (pointed out in the source below) that decrease mortality, but human mortality in child-birth is still much higher than other mammals.

Source: https://www.quora.com/Why-hasnt-evolution-gradually-reduced-...