The Mahabharata, an Itihasa, gives the answer perfectly.
The Shanti Parva (Chapter 342) of the Mahabharata.:
“ahamAtmA hi lokAnAM vishvAnAM pANDunandana tasmAdAtmAnamevAgre rudraM sampUjayAmyaham yadyahaM nArchayeyaM vai IshAnaM varadaM shivam AtmAnaM nArchayetkashchiditi me bhAvitaM manaH”
“I am the Soul, O son of Pandu, of all the worlds, of all the universe. Rudra, again, is my Soul. It is for this that I always adore him. If I do not adore that auspicious Shiva, the boon-giving Isana nobody would then adore my own self.”
Srimad Devi Bhagwatam Mahā Puranam 5:1:12-54. says thus.:
12-54. Vyāsa said :-...Again the Ṛṣi Nārāyaṇa is the part incarnate of Viṣṇu, and Vāsudeva is the part incarnate of the Ṛṣi Nārāyaṇa; hence what wonder is there, if Vāsudeva be seen to adore and propitiate Śiva? Śiva is the God of gods; and He is the Lord of all the causal bodies that exist; in the state of Suṣupti (deep sleep). In this respect, Śiva is the creator of Viṣṇu and Viṣṇu worships Him in this light. Rāma, Kṛṣṇa and others are all part incarnations of Viṣṇu; so there is no wonder if they worship Śiva......."
Atharvashika Upanishad 2:1.:
“The pranava (the sound of Om) makes all the souls to bow before it. It is the one and only one which has to be meditated upon as the four Vedas and the birth place of all devas. One who meditates like that goes away from all sorrows and fears and gets the power to protect all others who approach him. It is because of this meditation only that Lord Vishnu who is spread every where, wins over all others. It is because Lord Brahma controlled all his organs and meditated upon it, he attained the position of the creator. Even Lord Vishnu , parks his mind in the sound (Om) of the place of Paramathma (ultimate soul) and meditates upon Îsâna. , who is most proper to be worshipped. All this is only proper in case of Îsâna. ”.
The Main reason.:
Mahābhārat — Anūśāsan Parva- Chapter —18 , Verse-64.:
Vasudeva said., "...If Rudra be gratified with a person, he can confer upon him the states of eIther Brahma or of Kesava or of Sakra with all the deities under him, or the sovereignty of the three worlds. Those men, O sire, who worship Bhava even mentally, succeed in freeing themselves from all sins and attain to a residence in heaven with all the gods..."
~ Mahābhārata Anushāsana Parva Sec. XVIII.
Sathapatha Brahmana 13:6:1:1.:
"Purusha Narayana desired, 'Would that I overpassed all beings ! would that I alone were everything- here (this universe) !' He beheld this 'five days' sacrificial performance, the Purushamedha and took it and performed offering therewith ; and having performed offering therewith, he overpassed all beings, and became everything here. And, verily, he who, knowing this, performs the Purushamedha, or who even knows this, overpasses all beings, and becomes everything here."
Taittiriya Aranyaka 10:24:1.:
"Lord Rudra is the (Veda) Purusha."
Mahabharata 7 : 172: verse No. 74,78.:
By the grace of me (Rudra) you will become Nārāyaṇa with unending strength among the deities, humans and Gandharvas.
“Through my grace, none shall ever be able to cause thee pain by the weapon of thunderbolt or with any object that is wet or dry, or with any mobile or immobile thing. Thou shalt be superior to myself if thou ever goest to battle against me.'”
"The Krishna, due to his devotion to Supreme Lord Rudra, Has spread all over the universe (jagat vyaptham mahathmana viz became all pervasive Vishnu), Oh Bharatha, Oh king of kings, After making Lord Shiva pleased by his penance in Bhadrinath, He has attained the state of being more dear, Than all the worlds and all aspects of knowledge."
“In every new Yuga has Krishna (by such penances) gratified Mahadeva. In every Yuga has Mahadeva been gratified with the great devotion of the high-souled Krishna“.
“Kesava is that devoted worshipper of Rudra who has sprung from Rudra himself. Kesava always worship the Lord Siva, regarding his Phallic emblem to be the origin of the universe. In Kesava is always present that knowledge, in consequence of which he views the identity of Brahman with the: universe and that other knowledge by which the Past, the Present and the Future, the near and the remote, are all seen, as if the whole are before his eyes. The gods, the Siddhas and the great Rishis, adore Kesava for obtaining that highest object in the universe, viz., Mahadeva. The Lord Kesava always worshippeth Siva in the Phallic emblem as the origin of all creatures. The God having the bull for his mark cherisheth greater regard for Kesava”.
Nandi to Visnu: अस्य प्रसादलेशस्य लवलेशलवेन तु । ईदं विष्णुपदं लब्धम् त्वया नान्येन हेतुना ।।
By a smallest part of Śiva's grace you have attained this Viṣṇu position not otherwise.
(Sūta Saṃhitā ŚivaMahātmya Khaṃḍa —3:14). This is citied in Skanda Purāṇam and also cited by Vidyaranya Swami in his Sūta Saṃhitā Commentary!!
"With truth, purity, sincerity, resignation, ascetic austerities, vows, forgiveness, devotion, patience, thought, and word, I (Rudra) have been duly adored by Krishna of pure deeds. For this there is none dearer to me than Krishna".
I hope this clarifies all your queries. Prd..