Station 19, season 6, episode 10 suggested that Beckett might lose his captaincy, giving the series the chance to do what they should have already done: make Andy Herrera captain. Throughout its six seasons, Station 19 has seen many captains, but despite Andy’s best efforts, she has remained a lieutenant. There were multiple prior opportunities in which Andy could have been named captain, but Station 19 season 6 seems to be the perfect time to finally give Andy what she deserves and has earned.

https://screenrant.com/station-19-andy-captain-time-reasons/