The goal of the Bhagavatam is to glorify Krishna. This is important because Krishna is a special form of the Supreme Lord. That is why in the list of avatars given by Suta Goswami, he says Krsnas tu bhagavan svayam - the original Supreme Personality of Godhead is Krishna.
We may ask that why is Krishna superior to other forms of God because God is equal. Just like the example of a moon. The moon is one but sometimes its half moon, small moon (punima, amavasya etc) similarly the Lord will manifest a certain amount of qualities in every form so if we take all of these different forms we will see that some will manifest less qulities and some manifest less qualities just like the moon sometimes will manifest less light and sometimes more light.
It's not that other forms of the Lord are created and destroyed, they are all eternal (like Narsimhadev, Lord Rama, Visnu etc) but Krishna is purna bhagavan because He is manifesting all the 64 qualities.
What makes the beauty of Krishna special? It is the madhurya or sweetness in His form which makes Him special as it allows greater intimacy with devotees. Krishna's beauty is unrestricted and the form itself allows a greater relationship. The other forms of God will manifest dasya relationship (servitorship) and vatsalya rasa but Krishna has billions of sakhya rasa friends, vatsalya rasa, madhurya rasa and all are intimately connected with Krishna.Then is Krishna's madhurya flute. He has a very sweet flute and when He plays it, everyone who hears it gets madhurya prema. So in this way Krishna has these remarkable qualities which are very sweet not manifested in any other forms of the Lord and therefore He is put in the position of svayam bhagavan.
Beacuse He is manifesting such sweet qualities, that's why this form and qualities are very suitable for people of Kali-yuga. Because the people are always distracted in material enjoyment, are unintelligent absorbed in material things, they need some powerful attraction to pull them out of Maya. That's why Narada Muni told Vedavyasa to tell people to worship Krishna because telling them to worship devatas and gradually advance
from tamo guna to rajo guna to sattva guna (ignorance - passion - goodness) is difficult and that's why Vedavyasa wrote Bhagavatam in such a way that the pastimes of Krishna become prominment.
The only way we will understand why Krishna is swayam bhagavan is by appreciating His extra qualities and by appreciating His pastimes with His devotees. By hearing the pastimes of Krishna, we can appreciate the extraordinary qualities of Krishna. One of the main angas of Bhakti is to relish the Srimad Bhagavatam in association of devotees as this will attract us to Krishna. By that attraction we will lose our absorption in the material world. That is the mood of Narada Muni while giving advise to Veda-vyasa.
Even the atmaramas become attracted to Krishna. When Sukadeva heard about the qualities of Krishna, he forgot all about Brahman or impersonal realization. He was an impersonalist but he became an exalted devotee of the Lord. So exalted that he surpasses Vedavyasa. So when Bhagavatm comes out from the mouth of Sukadeva it becomes very sweet becuase His realization is very high.
- HH Bhanu Swami (Srimad Bhagavatam 1.1.1 Lecture)