Why are fossils only found in sedimentary?

Answer 1

Because sedimentary rocks are the only rocks that don't completely erode away the remains. If you had igneous rock with a skeleton in it, over a long period of time it would be gone because of all the processes.

Answer 2

Sediments are the ideal medium for fossil formation and preservation. An organism that is quickly covered by blowing sand, or sinks in a tar pit, or falls to the bottom of a lake or ocean and is covered by falling sediments from above stands a fairly good chance of winding up in the fossil record if the sediments become sedimentary rock. On the other hand, igneous rocks are formed from solidified magma; the odds of an organism or trace of an organism surviving exposure to molten rock is unlikely, but not unheard of. Metamorphic rock is igneous or sedimentary rock that has been transformed by heat and/or pressure. The heat and pressure may cause recrystallization, folding, or squashing of a rock that contains fossils, making the fossils unrecognizable due to distortion. Once again, fossils appearing in metamorphic rock are highly unusual, but not unheard of.