The answer lies at the very early stages of embryonic development:
"With white being the natural, unpigmented shade, as the embryo develops, the cells responsible for pigmentation move downwards from the vicinity of the spinal area. As these pigmentation cells migrate they encounter inhibitors, causing T-wave patterns that may form speckles or stripes. For solid colours, the cells responsible for pigmentation will migrate from the back and all the way to the front of the animal. In the case of two-tone piebald cats however, these cells stop before the journey is complete, creating the black-back-white-belly combination we see today."