Why is the spleen so much more likely to burst than the other organs?

Splenic rupture. The spleen is normally fist sized but it can swell to several times that size, unlike other organs which typically do not swell to this degree. The spleen acts as a filter, swelling is a normal response to certain disease states. The capsule or encasement of the spleen becomes very fragile in its swollen state, picture an over inflated water balloon. A sudden impact may cause rupture.
It's not. But it's fragile and trauma can make it break.