I would imagine that your own textbook has reasons that you are supposed to give. Or maybe your teacher gave them in class. Please check to be sure what we say on here agrees with those.
The usual explanation for why they were so rich is that their empire covered a large area, one which had a lot of resources including silver and gold.
The Incas eventually controlled somewhere between 6 and 9 million people in a variety of environments. An empire that large is sure to have a lot of resources that the rulers can take from the ruled.
The Inca civilization began around 1200 A.D. The people of the Inca's spread over a vast area as their civilization progressed. The civilization encompassed the Andes Mountains, the mountains of Peru, and went through-out present day countries of Ecuador, Bolivia, and Chile. The civilization was first established in Cusco. During the infancy of the empire deep mines of silver, gold, and other precious metals were discovered. In order to man the mines and develop other enterprises, the Inca's captured and used slave labor form the many smaller groups of native people.
Part of the establishment of wealth entailed the use of the laborers as well as the trade of their goods and labor among tribes. The Incas lived in regions that were abundant in rich soil leading to great food supplies. Having abundance in food supplies allowed the population to grow quickly and enabled them to quickly encompass a large region. It also enabled them to have an advantage over smaller tribal units. The Incas also had a common system of worship and an organized system of government.
Another reason that the Inca's were able to expand their population was that they were relatively advanced in medicine and technology for their day. They had physicians that used herbal medicines and even surgery to treat certain medical conditions. Their skills in this area reduced the deaths of infants, women, and men, and added to creating a larger society.