Diamond would take great exception to the idea that the Inca were "inferior." One of the main points of his book is that societies are not superior or inferior to one another. They may be more or less powerful, but this does not make one superior to another. Therefore, Diamond would prefer the phrase "easily conquered."
Some of the things that made the Inca easy to conquer were the things they lacked. In answering your previous question, I listed things the Spanish had. The Inca were easy to conquer in part because they lacked those things.
However, there were other things that made the Inca easy to conquer.
- Lack of immunity to European diseases. Smallpox epidemics had devastated the Inca, leading to a civil war that "left the Incas divided and vulnerable (p. 77)."
- The centralized nature of the Inca political system. Their system was so centralized that it fell apart when the emperor was killed.
- Lack of information. Because they had no writing, the Incas were unable to really know much about the Spanish. They could not get information from every Indian culture that had met the Spanish the way the Spanish could get information about Indians.
- Lack of exposure to the world. Diamond argues that the Incas were mostly exposed only to people like them. Therefore, they could not conceive of how different the Spanish were and how differently they might behave.