Why is succession slower on sand or bare rock than on previously vegetated soil exposed by a fire?
Succession would occur much more slowly on sand and bare rock than on bare soil. This is because of how much easier it is for things to grow on the soil than it would be on either the rock or the sand.
In order for succession to occur on rock or sand, it is generally necessary for some amount of soil to be built up. On bare rock, this would happen very slowly through weathering, through dust blowing on to the rock, through lichens, and other such slow mechanisms.
On bare ground, plants can sprout immediately and begin the process of succession.