I don't think you can legitimately ask "Why are humans bipedal?" because the why implies a cause of evolution, the more correct question would be "Why was being bipedal an advantage for the Homo genus?"
While this is entirely speculative, I suspect that early ancestors probably evolved very primitive hand-like paws for grasping and climbing; which was useful for defence (far fewer predators in trees than on the ground) and also opened up more opportunites for food. After this I would expect that we became an aquatic ape that (potentially) spent a lot of time in the water catching and eating fish, potentially while still living on land/in trees; and this would require our hands to work for walking, grasping and swimming. At this point, I would expect that our ancestors were potentially getting good at using sticks and rocks as tools, and this balanced out any loss of speed or strength of having more delicate arms/hands; and evolution began to favour individuals who were better tool users.
Over time, the more successful offspring were the ones who used tools more often; and being more upright made it easier to use tools.
