A book should begin by explaining the following four topics:

1. Sambandha: the relation (sambandha) of the book (vācaka) with its main subject (vācya).
2. Abhidheya: the means of attainment propounded in the book.
3. Prayojana: the completion state or end state brought about by the means stated in point 2.
4. Adhikarī: the eligibility of a person for studying the book.

These four topics are captured succinctly in the second verse of the first chapter, first canto, of the Śrīmad-Bhāgavatam which Śrī Jīva Goswami examines in Anuccheda 95 of the Bhagavat Sandarbha. There, he explains why the Śrīmad-Bhāgavatam is the best among the scriptures (1.1.2):

dharmaḥ projjhita-kaitavo ’tra paramo nirmatsarāṇāṁ satāṁ
 vedyaṁ vāstavam atra vastu śivadaṁ tāpa-trayonmūlanam
śrīmad-bhāgavate mahā-muni-kṛte kiṁ vā parair īśvaraḥ
 sadyo hṛdy avarudhyate ’tra kṛtibhiḥ śuśrūṣubhis tat-kṣaṇāt

The Supreme Lord is immediately arrested in the heart of the virtuous, as soon as they so much as desire to hear the Śrīmad-Bhāgavatam, which reveals the supreme duty of the self, devoid of all deception or compromise, and which is undertaken by the pure-heart, devoid of envy. The one truly-abiding Substantive, which grants auspiciousness and uproots the threefold miseries, is the truth investigated here in the Śrīmad-Bhāgavatam. So, what need is there for any scripture other than this beautiful Bhāgavatam, composed by the great sage Vyāsa deva?

The reason the Śrīmad-Bhāgavatam is supreme is that it attracts even Kṛṣṇa, who is the most attractive person in existence. We can glean the four topics mentioned above from this verse.

Sambandha: The main subject which the Śrīmad-Bhāgavatam discloses is the one Substantive (vāstavam vastu), Bhagavān Śrī Kṛṣṇa. That Kṛṣṇa is specifically this supreme vastu becomes obvious as the first section of the Śrīmad-Bhāgavatam unfolds. Anuccheda 51 of the Tattva Sandarbha explains this by citing verse 1.2.11:

vadanti tat tattva-vidas tattvaṁ yaj jñānam advayam
brahmeti paramātmeti bhagavān iti śabdyate

All those who have realized Absolute Reality (tattva) refer to that Reality as non dual consciousness (advaya-jñāna). This non dual consciousness is referred to as Brahman, Paramātmā and Bhagavān.

Abhidheya: The means prescribed in the book that discloses the subject is the supreme duty (paramo dharmaḥ), which Śrī Jīva Goswami notes is described in SB 1.2.6 – SB 1.2.13 (these two verses are reproduced below):

sa vai puṁsāṁ paro dharmo yato bhaktir adhokṣaje
ahaituky apratihatā yayātmā suprasīdati

The supreme duty of human beings mentioned herein is that which results in devotion to the Supreme Lord, who is beyond the ken of our material senses. (1.2.6)

ataḥ pumbhir dvija-śreṣṭhā varṇāśrama-vibhāgaśaḥ
svanuṣṭhitasya dharmasya saṁsiddhir hari-toṣaṇam

Therefore, O best among the twice born, the perfection of duties attentively executed by people according to their varṇa and āśrama lies in the satisfaction of Śrī Hari. (1.2.13)

Bhakti is the supreme dharma because it’s only purpose is to please Bhagavān. Other purposes like desires for mukti, or for any personal gain are completely rejected in the Śrīmad-Bhāgavatam (projjhita-kaitava).

Prayojana: The prayojana is the completion stage to be achieved by a sādhaka. The prayojana is expressed in the line:

sadyo hṛdy avarudhyate ’tra kṛtibhiḥ śuśrūṣubhis tat-kṣaṇāt: the Supreme Lord is immediately arrested in the heart of the virtuous, as soon as they so much as desire to hear the Śrīmad-Bhāgavatam,

The phrase ‘the Supreme Lord is immediately arrested in the heart’ signifies that the Śrīmad-Bhāgavatam has the potency to attract Kṛṣṇa, the most attractive person. As only pure bhakti attracts Kṛṣṇa (He says, bhaktyāham ekayā grāhyaḥ: I can be attained by one-pointed devotion, SB 11.14.21), this line identifies the prayojana as prema.

This is the main reason why the Śrīmad-Bhāgavatam is the supreme scripture: it attracts the most attractive Bhagavān Śrī Kṛṣṇa.

Śrī Jīva explains that the word Bhāgavata means that it establishes or reveals Bhagavān, and Śrīmat means that it has the potency to do so in its essential nature. Just as fire possesses natural burning capacity, Śrīmad-Bhāgavatam has the natural potency to attract and reveal Kṛṣṇa.

Adhikarī: The adhikarī for this book, or the person who is eligible to study the Śrīmad-Bhāgavatam and benefit from it, is referred to by the words nirmatsarāṇāṁ satāṁ, or the virtuous who are devoid of envy. Śrīdhar swami defines envy as intolerance of the value found in others (परोत्कर्षासहनं). In addition, Śrī Jīva notes that the world nirmatsara includes those merciful people who do not kill animals (पश्वालम्भने दयालूनाम्). Overall, the main qualification for realizing the prayojana of prema is lack of envy, which is also expressed by the word kṛtibhiḥ. Only in such people is the Supreme Lord immediately arrested in the heart, as soon as they so much as desire to hear the Śrīmad-Bhāgavatam.

Summary:

The book Śrīmad-Bhāgavatam has the following four features to it:

1. Sambandha: The main topic is Bhagavān Śrī Kṛṣṇa, and this book reveals Him. This is why Śrīmad-Bhāgavatam is the topmost scripture.
2. Abhidheya: Uttama-sādhana bhakti.
3. Prayojana: Bhāva or prema.
4. Adhikarī: One who sees value in all beings, i.e. is free from envy.