Why did Park Geun-Hye’s impeachment lead to a snap presidential election in 2017?

Park Geun-Hye’s impeachment led to a snap presidential election in 2017 because the South Korean constitution mandates that a new election must be held within 60 days of a president's removal from office.12

Following a major influence-peddling scandal involving her close friend Choi Soon-Sil, the National Assembly voted to impeach Park on December 9, 2016. On March 10, 2017, the Constitutional Court unanimously upheld this decision, officially removing her from the presidency. This removal stripped Park of her presidential immunity, and in accordance with constitutional requirements, the snap election was scheduled to determine her successor. The election took place on May 9, 2017, resulting in the victory of Moon Jae-In.132