While no one has an authoritative answer, anthropologists have long theorized that early humans began walking on two legs as a way to reduce locomotor energy costs.
In the first study to fully examine this theory among humans and adult chimpanzees, published online July 17 in the Proceedings of the National Academy of Sciences, researchers have found that human walking is around 75 percent less costly, in terms of energy and caloric expenditure, than quadrupedal and bipedal walking in chimpanzees.
That energy savings could have provided early hominids with an evolutionary advantage over other apes by reducing the cost of foraging for food.
Of course, that still doesn't explain why it become fixed when and where it did. And why is bipedalism so rare if it's so much more efficient? (probably because running on four legs is quicker)