Here's why:
* Intransitive verbs do not take a direct object. They express a complete action without needing to act on something else.
* Transitive verbs require a direct object to complete their meaning.
In this sentence, "truth" is the subject and "triumphs" is the verb. There is no object that "truth" is acting upon. The sentence expresses a complete thought without needing to explain what "truth" is triumphing over.