Why does acute rheumatic fever target the mitral valve to cause mitral valve stenosis?

It can target any va: Rheumatic fever is no respecter of valves. Mitral valve, followed by aortic valve involvement occur. Rarely is the pulmonic or tricuspid valve involved. Of course, joints, skin (rash) nodules and various neurologic sequelae are also found.

Pancarditis: The characteristic involvement of the heart by ARF attacks all layers (endocardium, myocardium, epicardium, pericardium, etc.). Why the mitral valve is damaged to a greater degree than other valves is not known. The sequence is mitral, aortic, mitral and aortic, tricuspid and very infrequently, pulmonic.