Krishna ! Guruvayoorappa ! Dear Bhaktas,
Before returning to His permanent abode Vaikuntam , Bhagavan Krishna informed His main disciple Uddhava , about His imminent departure and the forthcoming tough "Kaliyuga" ( age of deterioration) . As requested by Uddhava, Bhagavan infused His "Chaithanyam" ( aura) in to the four armed Deity of Maha-Vishnu ( which was worshipped in Dwaraka) . This Deity incarnation of Krishna is for resolving all ills of Krishna-Bhaktas in Kaliyuga . As desired by Bhagavan, the Deity was later installed at Guruvayoor .

Sree Krishna is the "poornavatar" ( complete incarnation) of Maha-Vishnu . This particular incarnation is the one most easily accessible to the Bhakta . That is why Krishna is worshipped as Maha-Vishnu in most of the Vishnu temples . Of course, there are specific Maha-Vishnu temples like that of Sree Padmanabha temple in Trivandrum . Bhagavan's Divine will exists behind the creation of each Vaishnava temple . Man is just an instrument to implement and not the creator .

May Bhagan Krishna bless you all !!!
Guruji108
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Hare Krishna!

In my limited Gaudiya understanding, Maha-Vishnu is an expansion of Lord Krishna. Maha-Vishnu is the incarnation of Lord Krishna in the aspect of Creator, then divided into Kshirodakashayi-Vishnu (Dweller in the heart of every living being, Paramaatmaa, etc.) and Garbhodakashayi-Vishnu (Creator and Dweller in the Macrocosmic forms, universes, etc.).

Worship of Krishna is worshipping all His incarnations and demigods. He is fully sufficient.

Jai Sri Krishna!

There are a few reasons to that:
1. Madhva himself found a deity of Krishna in Udupi, that became the center of attraction for the whole town. "poornavatar" for him meant that he has all the qualities of Narayana.
2. Sri sampradaya started with alvars, some of whom, such as Kulashekharalvar, were worshiping Vishnu and Krishna on the equal basis and did not distingush philosophically who is first.
3. Krishna worship is accepted to be earlier then Vishnu worship, such as in Krishna-Vasudeva traditions.
4. Krishna is a preference of worship for many Vishnu bhaktas, because of personal preference.

Taking four of the above onboard it will explain to you the reasons.

On the Doctrine of Avatara

Why was Saivism the most oldest and withstood the Test of Time.

They didn't believe in the Doctrine of Avatara

During the test of time they were not praying to the Avatar's to be born and help them

Rather they themselves through each one has being blessed with the powers of god and stood against instead of seeing it and praying for gods

If we think we are the Warriors of god and by our self take courage to stand against the evil

God is One Almighty He need not take any Avatars we are his Warriors The Soldiers of His Kingdom,We need his blessings to Win Over the Evil Not him for every Single thing God Is not so cheap
(If you children's are fighting you need not take an avatar of a child and help him out ,you rather send your men to sort out the Problem

Because Mighty God,never comes He Sends His Men
we all are the AVATARS and Warriors of God His Sena's and Ghana's

Har Har Maha dev