GPU are massively parallel and are extremely good at solving computation that don't require moving large amounts of information or making large numbers of decisions. CPUs are somewhat parallel and are extremely good at solving computations that require moving large amount of information and making large numbers of decisions. Hashing, as it happens, doesn't require moving large amounts of information and doesn't require making large number of decisions. So GPUs win.

A GPU is like a CPU with 1,000 very limited cores.