It's just a limit for the construction of the diaphragm. Either the the specific construction makes it impossible to close further, or the manufacturer has chosen that point as the limit to preserve quality.
The diaphragm is usually made out of blades with a curved edge. The curve is formed so that it makes a hole that is as round as possible at the biggest settings. When the hole gets smaller, it goes from being almost round to being almost formed as a polygon. For a six-blade diaphragm for example it would approach the shape of a hexagon. The un-roundness of the hole reduces the image quality somewhat, so you don't want to get too close to the polygon shape.
Also the optical effect called diffraction reduces the image quality when the hole gets too small. When that happens depends mainly on the sensor size and resolution.