Walking of two legs apparently requires less energy.

The researchers found that human walking used about 75 percent less energy and burned 75 percent fewer calories than quadrupedal and bipedal walking in chimpanzees. They also found that for some but not all of the chimps, walking on two legs was no more costly than knucklewalking.

Less energy consumption would mean less food intake, allowing for tolerance of a more varied climate. This is an aspect of the forest/fringe hypothesis that early hominids exploited the fringe area that the precursors to the chimpanzees would not and, therefore, evolved along a different path.