Here's why:

* Deltas form at the mouth of rivers: Deltas are created when a river flows into a larger body of water (like a lake or ocean) and deposits its sediment load.

* Sediment is eroded from upstream: The sediment that makes up a delta comes from the erosion of rocks and soil upstream along the river's course. This can include mountains, hills, plateaus, and other landforms.

* Volcanic material is different: While volcanic eruptions can deposit ash and rock that can be transported by rivers, this is not the primary source of material for most deltas. Volcanic material is typically different in composition and texture from the sediment eroded from other landforms.

In summary: Deltas are formed by the deposition of sediment eroded from the surrounding landscape, not directly from volcanic activity.