after the process instance is completed, it is removed from the runtime database.
The community edition Cockpit shows only processes from the runtime database tables.
The history view of the completed processes is a feature of the enterprise version: https://docs.camunda.org/manual/7.14/webapps/cockpit/bpmn/process-history-views/.
But recently a cockpit plugin with a simplified history view was release by @datakurre: Minimal Cockpit "history plugins" for Camunda >= 7.14.0
Hope this helps, Ingo