The Cenozoic era is the shortest and most recent era of all, the one that still lasts and we live in. The major reason why the scientists have by far the most knowledge about this era is exactly the fact that it is the most recent. Because it is the most recent era, the rock layers are better preserved, and in much lesser degree destroyed when compared to the previous eras. The fossils records are also much richer, as the fossils need longer periods of time to be destroyed, so there's an abundance of them. The other factor is that there a lot of remains in the most recent part of this era by the humans, which are also a nice representation of what has happened, how it happened, what were the living conditions etc.