Why did Madri bear the last two Pandavas and not Kunti?

Why did Madri bear the last two Pandavas and not Kunti?

Why did Madri bear the last two Pandavas and not Kunti?

Bharathi is a Chennai based writer and editor whose childhood passion for writing transformed into a career in this field. She is the author of the story "The fall of the first son" from the Unsung Valour and "Subhadra - The auspicious one" from Aryaa anthologies.
Rishi Kindama cursed Pandu with his dying breath that when he touched his wife, he would fall dead too. A heart broken and guilt stricken Pandu gave up his royal pleasures, donated all the wealth he and his two wives had with them and began to live in the forests. After visiting many kshetras and spending a lot of time with sages, Pandu, and his two wives, Kunti and Madri, decided that they should ascend to heaven by walking up the Himalayas. However, a man without sons cannot move to the next world.
 
Kunti revealed to a grief stricken Pandu about her boon from Durvasa muni that allowed her to summon the Gods and bear sons from them. Now Pandu was very happy and he wished to get a son from the God of justice, Dharma. 
Yudhishthira was born, followed by Vayu putra Bheema and Indra putra Arjuna.
Pandu was not satisfied yet. He asked Kunti to bear more sons but she did not agree. She reminded him that dharma did not allow this. Now Madri, Pandu’s second wife, was very eager to become a mother too. She approached Pandu in privacy and told him her wish to bear sons.  

Pandu asked Kunti to share the mantra with Madri. Using the mantra, Madri summoned the Ashwini twins and bore twin children- Nakula and Sahadeva.
Kunti’s refusal to bear more than three, as dharma dictated, and Madri’s desire to have sons resulted in the first wife bearing the elder three sons of Pandu and the second one bearing the younger two.