You can find this answer in the part that is headed with the italicized words "How did Pizarro come to be at Cajamarca..."
The basic idea here is that both Spain and the Inca Empire had centralized forms of government. Having this, however, helped the Spaniards and hurt the Incas.
Because Spain had a centralized government, Spain was able to pay for the ships that brought Pizarro to South America. The centralization allowed the government to send soldiers to South America and to make sure that they were equipped to fight.
By contrast, the Incas' centralized government hurt them because it depended totally on the monarch. Once he was captured, there was no leadership and everything fell apart for the Incas.