Carbon has exceptional properties which has enabled designers to produce machines of such performance and durability that would have been quite impossible with any other material.
It’s unique self lubricating properties enable the machine to run for thousands of hours with little commutator wear.
High contact resistance of carbon enables motors to operate sparklessly over a wide range of currents and speeds.
One reason is the high melting temperature of carbon. Electrical arcs produce very high temperatures- High enough to melt metals. Carbon is less prone to high temperature effects and associated impacts such as fusing or welding to adjacent metal parts.
It has a very high melting point (~3500degC) compared with other conductors. Brushes made with copper or steel would wear out faster and so can never be used.
Naturally with so many sources of raw materials available, many mixtures of carbon can be made which, with variations in subsequent treatments, make it possible to give the final products a whole range of characteristics to suit the widely varying requirements of different machines.
These advantages that carbon offer for brush material are so very important that it is virtually the only material best suited for this application.