Here's why:

* Sedimentary Rocks and Grain Size: Sedimentary rocks are formed from the accumulation and cementation of sediments. The size of these sediments is a key factor in classifying the rock.

* Conglomerate's Characteristics: Conglomerate is made up of rounded gravel-sized fragments (larger than 2 millimeters) that are cemented together. This makes it the coarsest common sedimentary rock.

* Other Coarse Rocks: While conglomerate is the coarsest, some other sedimentary rocks can have large grains, like breccia (angular fragments), but these are often classified based on their angularity rather than solely their size.

Let me know if you have any more questions about sedimentary rocks!