The most accurate response to why Quechua is still present in modern-day South America is:
It was imposed on every tribe conquered by the Incas.
This is because the Incas established Quechua as the administrative and lingua franca of their empire, leading to its widespread use across different regions and communities. Additionally, while other factors, such as cultural preservation and identity, contribute to its continued presence, the Incan influence was a significant reason for its initial spread and ongoing relevance.