The Inca culture was one of the most developed in pre-Columbian America. Among their achievements, was their architectural ability. Inca structures were made of stone that fit together, with extraordinary stability. The buildings have survived for centuries, with no use of mortar, in areas that are prone to earthquakes and volcanic activity.
The Inca also developed calendars based on astronomical observations. Astronomers understood equinoxes, solstices and zenith passages. Numerical information, particularly for administration, was stored in <em>quipu</em> strings.
In terms of art, Inca ceramics were highly advanced, although most of their silver and gold work was melted down by the Spaniards.
They were also advanced in medicine, performing skull surgeries successfully and with high survival rates.
The answer is the wounded knee
There were smarter, more qualified people working for the government which lead to a stable government.
