Why does mono only affect the spleen?

It doesn't. Mononucleosis is an infection by the epstein-barr virus. The infection is fought off by the body's immune system, composed of white blood cells and other proteins. The spleen is similar to a giant lymph node and will swell up as it fights the virus, as do the neck lymph nodes. The spleen is mentioned because it's location and thin covering makes it vulnerable to trauma and rupture with hemorrhage.
Affects whole body. Mono, caused by ebv virus, affects the whole body, not just the spleen. The spleen, lymph nodes (glands), and tonsils are all part of the immune system, and all get quite enlarged as they react to the ebv infection. Sometimes the tonsils get so big that a mono patient has trouble swallowing.