Thomas Gibson brought Criminal Minds Supervisory Special Agent Aaron Hotchner to life for 11 years, providing an anchor as other cast members came and went. To many fans, Hotch's strength in the face of horrific events was a necessary source of calmness, which made his quiet departure in the 12th season that much more unexpected.
Source:https://www.cbr.com/criminal-minds-why-aaron-hotchner-thomas-gibson-left/