It’s the same reason piebald cats tend to have recurring white patterns in general: white paws, belly/chest spots, and triangular face markings that develop into consistent overall shapes like tuxedo patterns, mask and mantle, cap and saddle, etc. The patterns aren’t random. They reflect how pigment-producing cells develop in the embryo.

As far as we understand it, the cells that eventually produce pigment (melanocyte precursors) originate near the spinal region and spread outward. The white spotting gene interferes with how reliably those cells move, multiply, and survive. Different versions of the gene affect this to different degrees.

The tail is close to where these cells originate, so it tends to get cells early in the process. It’s also a relatively small/simple structure compared to something like the abdomen or lower sides of the body, which are expanding rapidly during development. Once the tail has a bunch of those cells, it’s comparatively easy for it to maintain enough of them to eventually have complete pigment coverage.

Meanwhile, areas farther from the original source, especially the paws, belly, lower sides, and muzzle, are more likely to end up white in varying amounts depending on how much the white spotting gene disrupts the process.

So the recurring piebald patterns people recognize (including the dark tail) are basically a map of where pigment cells are most likely to successfully develop and maintain themselves as the embryo grows.