Why does the Basque word for an intransitive subject lack the -k ending?

The Basque language is of the ergative type, meaning it uses a specific case marker to denote the agent of a transitive action. The suffix -k is used exclusively to mark the subject of a transitive verb, identifying it as the agent performing the action.1

In contrast, the subject of an intransitive verb is not distinguished from the object of a transitive verb. Because these roles do not function as agents of a transitive action, they do not receive the ergative -k marker and therefore have no overt grammatical mark.1