Why is the spleen such a fragile organ?
Several factors are involved.
The organ is quite profusely filled with blood and is extremely close to the abdominal wall.
And the filled intestines, stomach, and lowering of the diaphragm on inhalation push the spleen even closer to the inner surface of the abdomen.
Therefore, when a blow to the right subcostal area occurs, there is a high probability it will hit the organ directly.