There are a number of reasons. - The dough is not kneaded in this recipe, and one of the ways in which gluten is formed is by stretching the dough by kneading it. To enable gluten to form in this recipe, leaving the dough to proof overnight gives the yeast the time it needs time to interact with the flour and liquid to form the gluten. - The oats and seeds tend to make the dough heavier and the yeast needs more time to generate sufficient carbon dioxide to form the bubbles that are needed to make the dough rise.