What caused th Inca empire to decline?

The conquistadors consolidated their power over the whole Andean region, repressing successive Inca rebellions culminating in the establishment of the Viceroyalty of Perú in 1542. 1572 saw the fall of the last of the Incas and the end of their resistance in Vilcabamba. Their civilization ended at that time, but cultural traditions remain in some ethnic groups such as the Quechuas and Aymara people. ( Wikipedia ).