There are a few reasons. Smaller ball size means more surface area. When the paste flows, the surface tension of the solder makes it cling to the part and pad,

There are a few reasons. Smaller ball size means more surface area. When the paste flows, the surface tension of the solder makes it cling to the part and pad, and moves the part to align with the pad. With more surface area, you can start to get solder that clings to itself, making solder balls outside of the pad, called balling. This wastes solder and makes conductive components that are effectively contamination, they now have to be cleaned off, you can't let them remain to roll around and potentially make a short. I had issues with smaller balls clinging to stencils; i think the price is also higher. I think there is a rule of thumb for choosing solder ball size, and it was something along the line of 5-8 balls should fit in the width of the smallest solder aperture. This will give good performance according to the component size without too much balling or other flow issues.