Why do some rocks contain fossils while most do not?

Fossilization is a rare process that usually requires rapid burial in sediment, low oxygen, and the right chemistry for mineral replacement. Hard parts like shells, bones, and teeth are much more likely to survive than soft tissue. After burial, the remains must avoid being eroded away or transformed too much by heat and pressure. That is why fossil-bearing rocks are common in sedimentary formations but rare in igneous rocks. Famous fossil beds, such as the Burgess Shale in Canada, preserve exceptional detail because burial conditions were unusual.