Because, in the last devastation, knowledge of the Absolute Truth was altogether erased from the universe, Brahma desired that the same knowledge again be renovated; otherwise there would be no meaning in the creation
SB Canto 2
To create different planetary systems I had to undergo austerities and penance, and the Lord, thus being pleased with me, incarnated in four sanas (Sanaka, Sanat-kumāra, Sanandana and Sanātana). In the previous creation the spiritual truth was devastated, but the four sanas explained it so nicely that the truth at once became clearly perceived by the sages.
The Viṣṇu-sahasra-nāma prayers mention the Lord's name as sanāt and sanātanatama. The Lord and the living entities are both qualitatively sanātana, or eternal, but the Lord is sanātana-tama or the eternal in the superlative degree. The living entities are positively sanātana, but not superlatively, because the living entities are apt to fall to the atmosphere of noneternity. Therefore, the living entities are quantitatively different from the superlative sanātana, the Lord.
The word san is also used in the sense of charity; therefore when everything is given up in charity unto the Lord, the Lord reciprocates by giving Himself unto the devotee. This is also confirmed in the Bhagavad-gītā (4.11): ye yathā māṁ prapadyante. Brahmājī wanted to create the whole cosmic situation as it was in the previous millennium, and because, in the last devastation, knowledge of the Absolute Truth was altogether erased from the universe, he desired that the same knowledge again be renovated; otherwise there would be no meaning in the creation. Because transcendental knowledge is a prime necessity, the ever-conditioned souls are given a chance for liberation in every millennium of creation. This mission of Brahmājī was fulfilled by the grace of the Lord when the four sanas, namely Sanaka, Sanat-kumāra, Sanandana and Sanātana, appeared as his four sons. These four sanas were incarnations of the knowledge of the Supreme Lord, and as such they explained transcendental knowledge so explicitly that all the sages could at once assimilate this knowledge without the least difficulty. By following in the footsteps of the four Kumāras, one can at once see the Supreme Personality of Godhead within oneself.