Pregnancies at the zoo can be a tricky thing to manage, especially if the animal in question is a high-profile one. On one hand you have the keepers who, while doubtlessly super-excited, tend to be cautious, more conservative in their approach. Giving birth can be risky business for the mother, and neonatal animals are very vulnerable. Many zoos keep newborns hushed-up until they are big enough and healthy enough to go out and on display. The decision is made to not call too much attention to the family until everything is a little more stable. (This varies by situation, of course - I've seen some zoo mothers give birth on display and then go calmly on with their day as hundreds of guests watch baby roll around on the ground).
Administrative and marketing folks, on the other hand, are often ready to start beating the drum and announce the news to the world. Dallas Zoo just took that approach to an extreme when they invited the entire Internet to watch one of their female giraffes, Katie, give birth online... live.