Question

Why are the last 570 million years of Earth's history divided into three different eras while the preceding four billion years comprise only one era?

Answer 1

Today they are divided into many eras (see below).

Explanation:

From today, going back to the formation of the Earth these are all the eras:

Cenozoic .................. 66 million years ago to present day
Mesozoic ................. 252.17 to 66 million years ago
Paleozoic ................. 541 to 252.17 million years ago
Neoproterozoic ...... 1,000 to 541 million years ago
Mesoproterozoic .... 1,600 to 1,000 million years ago
Paleoproterozoic .... 2,500 to 1,600 million years ago
Neoarchean ............. 2,800 to 2,500 million years ago
Mesoarchean .......... 3,200 to 2,800 million years ago
Paleoarchean .......... 3,600 to 3,200 million years ago
Eoarchean ................ 4,000 to 3,600 million years ago
Hadean Eon ............. Formation of Earth to 4,000 million years ago

Originally, everything below the Paleozoic was one period called Precambrian, and the divide was because at the beginning of the Paleozoic marks the appearance of hard shelled animals.

Here is the wikipedia link in case you want to dig further into each era: https://en.wikipedia.org/wiki/Era_(geology)