Here's why:

* Compression fossils form when an organism is buried under sediment and the weight of the sediment compresses the organism, flattening it. This leaves an imprint of the organism in the rock.

* Leaves are thin and delicate, making them ideal for forming compressions.

* The organic material of the leaf can sometimes be preserved, creating a carbon film that shows the leaf's detailed structure.

Other types of fossils are less likely to form from leaves:

* Casts and molds usually occur when the original organism decays, leaving a hollow space that is filled with minerals. While leaves can form molds, casts are less common.

* Trace fossils are the preserved evidence of an organism's activity, like footprints or burrows. Leaves don't typically leave traces like this.

* Petrified fossils occur when organic material is replaced by minerals, preserving the original shape. This process is less common for leaves.