The geologic scale is more detailed in the phanerozoic than in the previous eons because in this eon there's much more activity on the Earth, from a perspective for the life on it and its development. The evolution of life, the emergence of new species, and the dying out of them were all factors that contributed to making a more detailed scale, and usually an era ends with the dying out of big portion of the life on the planet, so called mass extinctions.