Radioactive decay of long lived elements in the core. There's a square cube relation between surface area and volume. Smaller planets have much more surface ar

Radioactive decay of long lived elements in the core. There's a square cube relation between surface area and volume. Smaller planets have much more surface area compared to volume than larger planets. So it takes a lot longer for heat to escape the core. While smaller planets can radiate their heat away relatively quickly, in the Earth it just builds up. Venus is of comparable size to Earth. But we don't know very much about Venus' geology. We know that there isn't any active vulcanism on Venus, its crust is all approximately the same age, and is (in geologic and astronomical terms) relatively young. The speculation is that the hot blanket of Venus' atmosphere actually keeps something of a lid on things. The magna isn't so much hotter than the atmosphere that vulcanism is an efficient means of letting off heat. So it just builds up for millennia (hundreds of millions of years? I do not recall the timescale.) until it reaches a critical temperature where most of the entire surface begins melting/venting/erupting. We can't prove it to any significant extent, but it's the model that best fits the evidence.