Such things happen naturally in a grand religion with scriptures written at various centuries across a time period of some 3000 – 4000 years.
As our Rishis have pointed out, the quality and caliber of people deteriorate from one yuga to another — people were at their best of dharma and spirituality in Satya Yuga and gradually deteriorate to their lowest level at Kali Yuga.
Accordingly, the capacity for the people to grasp religion and spirituality deteriorated in contrast to knowledge of science and technology advancing century by century! The more the head grows, the weaker the heart becomes.
Puranas too must have undergone deterioration accordingly. Sanskrit Pundits and scholars with highly religious bend of mind, who had access to ancient scriptures of Puranas too were subject to different ideas about God and they too might have got compartmentalized to sects like Shaivism, Vaishnavism etc. Accordingly, eulogizing one God over other and undermining one God over other might have started by them by writing new Purnas/ editing/rewriting/ inserting fresh supplements to established puranas with their own concoctions, intentional distortions and manipulations.
Vyasa and 18 Puranas – a critical view
Vyasa Maharshi (Krishna Dvaipayana Vyasa) is considered an Avatar of Lord Vishnu and he is credited with compiling all the 18 major puranas. He is also considered a Chiranjivin (immortal) – ever living. He is also credited with compiling Vedas into four (Rik, Yajur, Sama and Atharva Vedas). He was the author of the epic Mahabharata too. He is also said to be the author of Brahma Sutras.
If we take Vyasa’s Mahabharata and his Bhagavata Mahapurana, there are several variations in the happenings of events post Mahabharata war that appear in both the scriptures. While in Mahabharata the five Pandavas were described to be people who were in the clutches of their attachments and aversions till the end of their lives and they were mentioned to end up in Heaven. Same case with king Parikshit too. It means they were all like ordinary mortals who were destined to go through further birth and death cycles. On the other hand, Srimad Bhagavatam mentions them to become earnest seekers at their fag ends of life and attain moksha by Self-realization.
Puranas (as well as the Itihas Mahabharata) contain several stories of Gods, Devatas, Asuras, great devotees, great emperors, historical events and lineages of kings, description of geographies, theories about creation and so on. Many of them get repeated in various puranas, in different versions and variations. Naturally, the question arises how there are so much variations and distortions, if the author or all the puranas is one and the same Vyasa.
Some historians say that the time periods of writing Mahabharata, Brahma sutra, Bhagavata Purana etc are different, spreading across several centuries. Krishna Dvaipayana Vyasa (of Mahabharata) and Badarayana Vyasa (of Brahma sutra) could be two different personalities, according to some of them.
Or, it is also possible that Vyasa being a title, there could have been so many descendents of Vyasa (off Krishna Dwaipayana Vysasa) who, in subsequent centuries edited/ re-wrote original versions, wrote newer ones as per their own interpretations and and still maintained the authorship to be Vyasa (in general).
There is also a possibility that so many other scholars wrote / compiled Puranas in various centuries, but for the sake of gaining authenticity, they projected as if every Purana originated from Vyasa/written by the same Vyasa!
Different tastes and affiliations, different Puranas
According to Hinduism, God appears in the form in which an ardent Devotee does tapas (extreme austerities) to get the vision of God. Naturally, if a Shaiva (devotee of Lord Shiva) longs to have the vision of His God, God appears to him as Shiva. Such a devotee, by the strength of his personal realization becomes convinced that the Ultimate God is Lord Shiva and his mindset may get so firm that God could not be in the form of Vishnu or Brahma.
Thus, saints and gurus chose such of those Puranas that matched with their own experience, liking, taste, temperament and mental leaning as “the most authentic” and discounted other puranas. Thus Shiva Purana, Linga Purana etc were projected as authentic ones by Shaivas,while Vaishnavas stood by Vishnu Purana and Bhagavata Purana. Shaktas eulogized Markandeya Purana, Devi Mahatmiya etc.
It is perhaps in the same way that the pinnacle of Advaita experience (philosophy) of Upanishads got diluted to suit the capacity of intake of spiritual seekers who could not attain that level of oneness with Brahman. That’s how Vishishtadvaita and later Dvaita came up as alternative interpretations of scriptural truths, evolved based on the respective Acharya’s spiritual experiences with reference to God. It helped people by and large to get connected to their personal God and religion in a way accessible and practicable by them and also gave them the assurance that they are in tune with the scriptures.
Srimad Bhagavata Purana is considered the greatest of all the puranas because it contains all the three shades (Advaita, Vishitadvaita and Dvaita) in its stories, discourses, prayers and parables. It is one Purana that can be said to bridge both Bhakti (devotion) and Jnayna (Knowledge) in a wonderful way.
Only after the advent of Itihasas and Puranas, the practice of worshiping Avatars (like Rama, Krishna, Narasimha etc) came up in Hinduism, particularly suited for Kali Youga. That’s how Agamas/ Pancharatras too came up later in Kali Yuga establishing and formalizing worshiping of Gods through idols in formally consecrated temples for worship, which was not there at all at Vedic times.
It can also be noted that in Vedas’ Karma Kanda, worshiping of Devatas (Indra, Varuna, Agni, Savita, Vayu and so on) and satisfying them through yagas (fire sacrifices) was widely prevalent. Over passage of centuries and and after evolution of Buddhism and Jainism from Hinduism, fire sacrifices and offering cows and horses as sacrifice in Yagnyas stopped. Worship of Eswaras (Prime Gods like Shiva, Vishnu and Shakti) became more prominent leaving Devatas behind.
The Puranas, as part of Smritis have contributed in a big way to pave the way for this shift.