The quick answer to this question is that an animal or plant had to be very "lucky" to become a fossil, and even luckier to have its fossil remains discovered. Fossilization is an infrequent occurrence that is highly dependent on chance. In the past, like today, the remains of most organisms were eaten by animals, consumed by microorganisms, or weathered away. Only dead organisms that are buried in sediment quickly can escape these destructive natural processes and become fossils. After remains have been buried and preserved, they may still be destroyed by geological processes, or exposed and weathered away before people can find them.
Organisms that were very rare in their environment might never have been fossilized simply because the odds of preservation favor more numerous organisms. Animals with small and delicate bones, such as small birds and amphibians, would be less likely to be preserved - and discovered - than larger organisms with tougher bones. Soft-bodied organisms like worms are even more poorly represented in the fossil record because they had no hard parts that could resist decay. We usually learn of their existence only if we find fossilized burrows, or trackways or impressions left by their bodies in soft sediments.
Scientists studying ancient ecosystems such as this one try to collect fossils from as many types of organisms as possible, but they never expect to find fossil evidence of everything that lived there.