Mosses need tow things to grow: shade and water. Moss doesn't have the same root system grass does, so it can grow on top of dense soil that chokes out most grasses. Grass is a sun-loving plant, which gives moss an edge in the shade. Moss typically reproduces through rhizomes and spores, meaning raking it away without killing it won't solve your moss problem -- it's likely to grow back before grass can gain a strong foothold.
