Why do scientists know very little about life in the Precambrian eon?

Pre-Cambrian life was basically soft-bodied - very few if any animals were left with bones, skeletons, or hard shells, so that very few fossils existed. It was also a long, good while ago - and natural activity on earth like volcanic action, subduction of continental plates, erosion, etc. washed out much of what was then the earth's crust.

The earliest planet history, Eon in the Precambrian means "before the Cambrian era." Initially used this old but still popular term to refer to the entire period of Earth's history before the discovery of the oldest rocks with identifiable fossils in them. They test and research ores from eroded and misshaped rocks in preparation for researchers to learn about Precambrian period.