K Sivan left ISRO after completing his tenure as the chairman of the organization. He served as ISRO chairman from January 2018 to January 2022, after which he stepped down and handed over the position to S Somanath. His departure was a normal transition after his term was completed, rather than a resignation under unusual circumstances. K Sivan's tenure included key missions such as Chandrayaan-2, which ended in a moon-lander crash, but also the approval and planning of the successful Chandrayaan-3 mission. His career at ISRO spanned decades, during which he progressed from a rocket engineer to the top post. Despite some controversies and criticisms during his time, including discussions around Chandrayaan-2's failure and leadership decisions, he left ISRO upon retirement age or term completion. In short, K Sivan left ISRO because his term as chairman ended and he retired from the position; he did not resign abruptly.