There's some chemical differences between thorium and uranium. Leach by water, eucompounds were widely dispersed. And having been scattered far and wide, eucompounds today are found as complex. Generally, dilute deposits containing a mixture of tetra, panta, and hexavalent uranium. Unlike uranium, tetravalent thorium, and it's constantly tetravalent, resist weathering. Thorium thus remained concentrated where it first wound up, with an easy reach.