Because IPv4 sucks! If you don't have enough publicly routable addresses then you are forced to use reserved ranges like 10/8. That means you'll get collisions, ie. multiple networks using the same addresses. With IPv6 you'd just get a real public IP address and all would be fine.

Edit: I feel bad for saying IPv4 sucks. It's one of my favourite pieces of tech and an astonishingly good one at that. It just doesn't have a big enough address space.