Type of parent rock. One gets different weathering products with different minerals, and different rocks contain different minerals. Soils thicken with time as weathering proceeds. It can take 10,000 years to create a good soil layer. The more weathering, the more soil.
Soils move downslope.
They are less thick on steeper slopes. This is the big one. Soils differ significantly between arid, temperate, and tropical climates. Basically it comes down to differences in the amount of rainwater, and differences in the amount of humus. If more rain, one also gets more plants, and potentially more humus.