Why is there Uranium in Earth's Crust?
Uranium is primarily lithophilic (it readily combines chemically with oxygen), but also is chalcophilic (it readily combines chemically with sulfur). It definitely is not siderophilic (it does not readily dissolve in molten iron). The first two means that uranium compounds are easily formed, which are of course much less dense than is pure uranium. That uranium is not siderophilic means that the mechanisms that led to the differentiation of the Earth had a lesser effect on uranium than it did on metals that easily alloy with iron.