“Another name for Lord Viṣṇu is Triyuga because there is no incarnation of Lord Viṣhnu in Kali-yuga. Indeed, this is the verdict of the revealed scriptures.”
“My Lord, You kill all the enemies of the world in Your multifarious incarnations in the families of men, animals, demigods, ṛṣis, aquatics and so on. Thus You illuminate the worlds with transcendental knowledge. In the Age of Kali, O Mahāpuruṣa, You sometimes appear in a covered incarnation. Therefore You are known as Triyuga [one who appears in only three yugas].”
Śrīla Śrīdhara Svāmī has also verified that Lord Viṣṇu appears in the Age of Kali but does not act as He does in other ages. Lord Viṣṇu incarnates for two purposes: paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām. That is, He comes to engage in pastimes with His devotees and to annihilate the demons. These purposes are visible in the Satya, Tretā and Dvāpara yugas, but in Kali-yuga the Lord appears disguised. He does not directly kill demons and give protection to the faithful. Because the Lord is not directly perceived in Kali-yuga but is directly known in the other three yugas, His name is Triyuga.