Here's why:
* Heap typically refers to a random, unorganized collection of something, usually small and loose objects. While stones can be in a heap, the term implies a lack of structure or purpose.
* Stones are usually more structured when grouped together. They might form a pile, a wall, a cairn, or a pavement.
Better alternatives for "stones" in a collective noun:
* Pile of stones
* Wall of stones
* Cairn of stones
* Pavement of stones
* Scattering of stones
The choice depends on the specific arrangement and context of the stones.