If the birth canal got too large, we wouldn't be able to retain our balance when standing.
See? Labor pains are a direct result of us being bipedal. We could just have smaller heads, but them we'd be dumber than a box of rocks. Anyway.. for some reason being bipedal is more of an evolutionary advantage than difficult births. So the best compromise we've been able to come up with is to have our kids born very early on in the development process.