Why does the mechanical advantage of an inclined plane decrease as you increase the angle of inclination?

At the two extremes of the angle of inclination you have no mechanical advantage. At an angle of 0 degrees, the plane is parallel to the effort force so it cannot raise the load. At the angle of 90 degrees, the plane is perpendicular to the effort force so all effort is directed into the plane. Between those two extremes, you have a machine. Divide the length (L) of the inclined plane by its height (H) to find the mechanical advantage. Notice that in the case where the angle of inclination is 0 degrees, H = 0. Division by zero is not allowed in math. (The mechanical advantage is undefined.) In the case where the inclination is 90 degrees, L = 0, 0/L = 0 (No mechanical advantage).