Efficiency. A lot of stuff is kept warm, or cooked utilizing the same equipment. In the mornings they use it to cook the sausages/eggs/bacon (because this is what people are looking for in the morning). In the afternoon/night they use the equipment to cook burgers etc.

While they definitely could serve both, its likely cheaper to simply have one at a time. Less equipment required.

Another thing, likely being that at night people don't want breakfast items (or far less) - so a lot of food would goto waste as they have it partially prepared before serving.