Why are the oldest fossils found in the oldest rock layers?

This is usually because when they first became fossils, they embedded into the ground. Over many years afterwards, more layers came to cover that layer and so on. Newer fossils would be embedded into newer layers. That's why the older ones tend to be found the deeper you look.

Answer: Well probably because of the fact that over time the rocks stack up on top of each other because of weathering and erosion and as they stack they get deeper and deeper, so fossils that were at the top one time will slowly sink as the time progresses.