The DMN Modeler follows a multi-editor approach. Therefore every has its own eventBus. Also see this thread for a short explanation: Are there two instances of EventBus's.
Therefore without specify the editor, the event handler won’t be recognized in most cases. You can find in the code base which editor fires which contextmenu event: https://github.com/bpmn-io/dmn-js/search?q=element.contextmenu&unscoped_q=element.contextmenu