The geological processes, which Mars lacks, are supposed to concentrate uranium, not strip it away. The actual geochemical explanation is complicated but long story short Mars is a geologically dead planet without much water that doesn’t like to form rock that can hold uranium ore.

Uranium concentrates in felsic (high silica) rock which forms under crustal recycling on Earth - the heat at the interface between the crust and mantle allows the Fe-Mg to strip away. Water dissolving uranium then injects into the felsic rock, the uranium precipitates out, and the rock later gets pushed up higher into the crust.

Mars has neither the active geology nor seemingly enough water to allow much felsic rock to form. Satellite surveys show the surface is covered with mostly basaltic rock (low silica, high Fe-Mg) with very small pockets of felsic. The processes that form these pockets are largely unlike those formed on Earth and don’t have the same geochemistry, especially without ample water.