Because of the diseases that the Europeans brought. The natives had not natural immunity to these diseases, so their large empire quickly fell as their numbers dwindle due to disease related deaths.
According to The Biography Channel, Spanish explorer Francisco Pizarro<span> was able to </span>defeat<span> the </span>Inca easily<span> because the Incans </span>were<span> embroiled in their own civil war and suffering from the smallpox epidemic, both of which had dramatically weakened the </span>Incan<span> empire.
