Isn't it to do with the human brain being too large so if we delivered our babies more fully developed the head would be too big? Being on all fours rather than walking upright would have allowed for a bigger head to be delivered I guess but maybe not big enough for the baby to be more independent.
Yes it's the price we pay for walking upright. There's a trade off between being upright and having big brains (which are needed to be upright in the first place) - evolution is a wonderful thing.
Love the kangaroo thing, their live births are barely formed and the mother has to lick a path through her fur to the pouch for the joey to follow. They start out about the same size as the mother's teat - so need big mouths!