PURPORT

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. In contrast to Sutapa and Prsni, they did not undergo severe penances and austerities to become the father and mother of Krsna. This is the difference between nitya-siddha and sadhana-siddha.