What was the major reason for the collapse of the Incas?

In 1527 Sapa Inca Huayna Capac died of smallpox, which was brought by the Spanish from Europe and had spread from Central America to South America, weakening the empire. Ninan Cuyochi, his eldest son and heir to the throne also died of smallpox, leaving no clear successor. According to Inca family tradition only the son of the Sapa Inca and the Coya, his legitimate wive, can become the next Inca emperor after the death of his father. Following this tradition Huascar was the next in line after the death of his older brother and was crowned Sapa Inca by the nobility in Cusco. Meanwhile in the northern administrative capital of Quito, his half brother, Atahualpa was considered a more capable warrior and proven administrator and was crowned as Sapa Inca by his supporters. However, as Atahualpa was the son of the Inca emperor and one of his concubines he had not legitimacy to the throne.