Radioactivity, in part, but also depth and pressure as well as convection to spread the heat around.
After thinking about this a bit, and doing some quick web research, it appears that the main lines of thought today say that the earth’s internal heat is mainly due to radioactivity, and the primordial heat from the formation of the planet (impacts, gravitational compression, etc.), in roughly equal amounts. (The percentages obviously cannot be quantified today; these are just estimates.)
As the contribution from radioactivity has declined through time, and the contribution from the formation of the planet cannot not change, it is likely that we had higher heat flow from the interior to the surface, thus more volcanic activity, etc.
Obviously, plate tectonics, mantle convection, etc. will complicate untangling the history of this heat flow, but it seem probable to me that a lot of the huge batholiths that date from 1.0 - 2.5 byp (especially the older ones) might be due higher heat flow from the interior of the earth, allowing more partial melting of crustal to form the huge granite implacements.
This sort of thing has always intrigued me, as I think that the deep geologic past, especially the bulk of Precambrian time, was a lot different from what we see today. The same processes occurred, but the drivers (heat flux from the interior, solar energy) and the rocks it worked on at the surface and the interior, are quite different from today. And the results were quite different from what we see today.