One reason is the high melting temperature of carbon.
The inductances of the energized windings, and the mechanics of a brush type motor or generator make arcing and sparking an unavoidable characteristic of the machine at the brush interface. Electrical arcs produce very high temperatures. High enough to melt metals.
Carbon has a very high melting point (~3500degC) compared with other conductors. Brushes made with copper or steel would wear out faster because of factors including:
1) The melted metal will break away from, or even be vaporized, leaving the brush with less material to do its job.
2) If the melted material doesn't vaporize or otherwise move away from the interface, it creates a mess for the brush. It can weld together bristles of the brush. It can build up material that interferes with the motion of the rotor.
3) As the metal melts in the presence of an arc, conductive material can be liberated, therby prolonging the arc, exasperating the wearing action.
Carbon is less prone to those high temperature effects and associated impacts to the generator operation.
Source : Dan Church
