Because the traditional habitat of the great Apes - the rainforest - was shrinking, the ability to walk more efficiently on hind legs would have been an asset for the first species of hominid, Australopithecus. The hypothesis that climate change drove the evolution of walking is one championed by Anthropologist Clive Finlayson, in his book The Improbable Primate.
Nowadays, I imagine, writing Genesis for a modern audience, we would see God arriving in a chauffeur driven stretch limo. Since Neolithic times God has been pictured on thrones and chariots, so why not an automobile?
Of course we are talking about origins here, so putting things in modern dress is reversing the sequence of events. In the beginnings of human evolution we didn’t have cars, we had our two legs, and they led us here over aeons of time. Nowhere else in the Bible is God depicted walking except in the story of our origins, in the book of Genesis.