Why do fjords have sills?
Fjords are formed as U-shaped valleys by the extreme erosion of glaciers. The glacier erodes large quantities of material that is transported by the ice. 

The glacier can erode deep under the sea level (remember how an iceberg only has 1/10 of its mass above the surface) and if the postglacial rebound is limited, the fjord will remain as an estuary. 

When the glacier slows down and withdraws, the sediments are dumped at the front of it to form a terminal moraine or sill. This is particularly pronounced if the front of the glacier had shrunken, growing again before melting due to environmental oscillations. 

The sills are usually deposited at the mouth of the fjord, before the water depth increases. However, the biggest part of the glacial debris is deposited at deeper water. 

You might also have shallow water in front of rias, were the flow slows down and sediments are deposited, but they will rarely form obvious sills as in a fjord.