Theo Ruiz was made Station 19’s Interim Captain, giving him a chance to do the role again, but he failed. When Captain Sean Beckett took leave to handle his drinking, Natasha Ross had to find an interim captain. She first asked Andy Herrera, but Andy turned it down because she believed she had earned being the permanent captain of Station 19, which she had. Ross couldn’t tap her boyfriend, Robert Sullivan, because of how it would have looked. Sullivan was the person who deserved the position, but Ross gave the role to Theo.
https://screenrant.com/station-19-theo-failed-captain-reasons/