From the Post:

The young Reds slugger agreed last fall to put his name in the pool for the U.S. team, but that was before Sean Casey was traded to Pittsburgh and Dunn was asked to move from left field to first base. With the position switch looming in spring training and plenty of talent already lined up for the U.S. team, Dunn passed along word to team officials that he’d rather concentrate on his Reds duties.