Why does the earth have more radon in some locations?
Radon nuclides are short-lived members of the natural radioactive decay chains of thorium and uranium. On a time scale that would be relevant for geology, you should better ask why some locations have more uranium or thorium. The distribution of radon concentrations in the ground air is a result of the distribution of uranium or thorium in the ground.

The main reason for the inhomogeneous distribution is igneous differentiation.  The original liquid magma might have been rather homogenous. But when the magma cooled down, fractional crystallization started. This removal of mineral precipitates also changed the composition of the remaining melt. Uranium and thorium ions are incompatible, i.e. they do not fit well in the crystal lattices of the silicate minerals that precipitate first. Therefore, they were accumulated in the remaining melt. Primary deposits of uranium and thorium minerals (such as pitchblende) were formed where the last parts of the cooled magma solidified.

Furthermore, secondary deposits were formed when uranium or thorium minerals were dissolved, transported and precipitated in ground waters. (This includes the nicely coloured and fluorescent uranyl ($\ce{UO2^2+}$) minerals.