Why do some beaches have pebbles instead of sand?

Pebble beaches are also know as shingle beaches.

Sand is the result of erosion and weathering of rocks over a long period. This causes them to become smaller and smaller until we call them grains of sand.

If the rocks are resistant to erosion and weathering, then they will simply not get smaller. This means they remain large and won’t get small enough to be grains of sand.