If you mean why do we mine as appose to quarry, then its depends on the geology of the mineral ore, and overlieing strata (layers of rock).
The pink mineral depost shown above is best exploited via a mine, or shaft, a kimberlite pipe or similar (diaminds) may be exploite in this way.
Grey = Shaft
Pink = Resource
Blue = Sky
Brown = Rock
A mineral deposit such as the one shown above is best exploited via open cast / quarrying.
Environmental Consultants Bristol