Because whenever Krsna appears on this earth He superficially needs a father and mother, Drona and Dhara, His eternal father and mother, appeared on earth before Krsna as Nanda Maharaja and Yasoda

Expressions researched:
"Because whenever Kṛṣṇa appears on this earth He superficially needs a father and mother, Droṇa and Dharā, His eternal father and mother, appeared on earth before Kṛṣṇa as Nanda Mahārāja and Yaśodā"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

Because whenever Kṛṣṇa appears on this earth He superficially needs a father and mother, Droṇa and Dharā, His eternal father and mother, appeared on earth before Kṛṣṇa as Nanda Mahārāja and Yaśodā.

SB 10.8.50, Translation and Purport

When Brahmā said, "Yes, let it be so," the most fortune Droṇa, who was equal to Bhagavān, appeared in Vrajapura, Vṛndāvana, as the most famous Nanda Mahārāja, and his wife, Dharā, appeared as mother Yaśodā.

Because whenever Kṛṣṇa appears on this earth He superficially needs a father and mother, Droṇa and Dharā, His eternal father and mother, appeared on earth before Kṛṣṇa as Nanda Mahārāja and Yaśodā. In contrast to Sutapā and Pṛśnigarbha, they did not undergo severe penances and austerities to become the father and mother of Kṛṣṇa. This is the difference between nitya-siddha and sādhana-siddha.