Since giant pandas eat mainly bamboo, which is hard to digest, they have a low metabolism that then results in them sitting around quite a bit. Since the mother has such a low metabolism, it makes her blood-oxygen level low. The baby has a better chance outside the mother to get the amount of oxygen that it needs. So this is one reason the baby is born so tiny.
The second reason is that the type of fatty acids that the baby needs can't be passed through the placental barrier. These fatty acids make the baby grow faster. The baby can get them through the mother's milk. Thus, it is better for the baby to be born early to get the fatty acids that it needs.
For the baby to get the oxygen and fatty acids that it needs, being born before it is big enough to take care of itself turns out to be a good thing for the baby. The baby will open its eyes around six to eight weeks old and may nurse for about eight to nine months. So the baby giant panda really needs its mother to survive!