The planet was probably a gas giant at one time, but when the local sun went supernova a few billion or so years ago, all of the planet's outer layers, which contained the lighter elements, were blown away. Any lighter stuff mixed with the remaining core soon boiled off. All that was left of the entire planet was a molten metal ball, and as it cooled, various metals froze out of solution with those of the highest melting points near the surface, and those of progressively lower melting points farther in. It was sort of like zone refining on a planetary scale. Except for that core, the entire planet is solid and not particularly hot.
Metals are much better conductors of heat than the rocky covering that Earth-like planets have.
All of the original heat has long since dissipated on New Kashubia, and the heat of decay from the more radioactive layers finds its way to the surface easily. Kasia, my throat is getting dry. Look, I'm not very thrilled about drinking my own reprocessed urine. It tastes warm and flat. Over the decades, the Japanese robots dug their way straight to the center of the planet to tap the mercury, and tunnels went off this central shaft at those levels that contained metals most in demand.