Why does white matter conduct impulses faster than gray matter

White matter is myelinated. This means it has a myelin sheath surrounded the neurone make up of Schwann cells with spaces between called nodes of Ranvier. At the nodes there is no myelin sheath.
The myelin sheath is an electrical insulator so the membrane on the neurone where it is myelinated can't be depolarised.
This increases the speed of conduction as Na+ (sodium ions), which depolarise the membrane and transmit the action potential, 'jump' from node to node by saltatory conduction. What actually happens here is that Na+ diffuse into the neurone, through the membrane at the nodes and then diffuse sideways inside the neurone to the next node. This increase in potential causes Na+ voltage gated channels in the neurone membrane to open, allowing more Na+ to diffuse in, which diffuse sideways to the next node etc. etc. The diffusion between nodes is much faster than depolarising the membrane all the way along and so the the increased length of the local circuit causes the action impulse to be conducted more quickly.
Axons with a myelin sheath can transmit nerve impulses around a 100 metres a second, whilst nonmyelinated neurones can only transmit at a speed of 0.5 metres a second.
Hope this helps.