Fossilization is a rare process, the vast majority of deceased organisms disappear without leaving a trace. Fossilization of an organism requires a unique set of circumstances so that it doesn’t just decay without a trace. These conditions include:

Some plants and animals are too small to be preserved as fossils.
Sediment deposition and accumulation is necessary for fossilization.
Animals with hard shells and skeletons are better represented in the fossil record.
High oxygen concentrations are required to prevent decomposition.
Most fossils are formed when a plant or animal dies in a watery environment and is buried in mud and silt.