Why has CO2 decreased in the history of the Earth?
The short answer is that most of the Earth's original allotment of CO2 got locked up in various carbonate minerals, largely calcite (limestone, marble, and chalk).  According to this article there is currently ~800 gigatons of carbon in the atmosphere today; there is ~39,000,000 gigatons of carbon locked up in calcite minerals. All that carbon was once in the atmosphere as CO2. Rainfall dissolved CO2 into H2CO3 (carbonic acid) which then reacted with calcium-containing minerals and then precipitated out in the world's oceans where they formed early calcite deposits.If not for this process the Earth's climate would not be too unlike Venus'.