The short answer is that the phanerozoic eon is more current therefore we have more material to study. We have fossils, rocks, and many more materials that scientists claim go back millions of years. That is a long time and a lot of items to classify. To make things easier, scientists broke the phanerozoic down to three major subdivisions of the eon called eras. The Archeon eon is so much older that we just do not have the materials to warrant extensive classification.
