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.