why does acidic character decrease down the group?

As we move down a group, the atomic size increases. Hence the ability of the element to lose the outermost electron also increases as the outermost electron becomes less strongly bound to the nucleus. Thus the electropositive or the metallic character increases as we move down a group. We know the more is the metallic character, more is the basic character of the element (metals usually form oxides that are basic in nature). So as the metallic character increases, the basic character also increases down a group. 

Therefore, we can say that on moving down a group, the acidic character decreases because the basic character increases.