The most accurate explanation for the abandonment of Machu Picchu by the Incan people involves a combination of factors, primarily the devastation caused by smallpox and civil war.
After the arrival of the Spanish conquistadors in the 16th century, the Inca Empire faced significant challenges, including the spread of diseases like smallpox, for which the indigenous population had no immunity. This led to a sharp decline in the population. Additionally, civil wars within the Inca Empire weakened its structure and resources.
While Spanish conquistadors did loot Incan wealth, including gold and silver, it wasn't the immediate cause of Machu Picchu's abandonment. Instead, it was the combination of disease, internal strife, and the resulting social upheaval that led to the site being left behind. The area was not completely abandoned immediately, but it fell out of use in the post-conquest period as Inca society collapsed and fragmented.
Therefore, the most fitting answer is that the population was devastated by smallpox and civil war.