The basic point here is, once a good human dies, he/she severs all of his/her ties with the world. So he/she is freed from earthly desire or MOH and attains salvation or MOKSH.
Then his/her soul finally reaches the stage where one's soul meets the Divinity itself and that is the ultimate goal.
Good people reach heaven as a reward of their deeds and live happily ever after.
But for Piya, her Salvation and Heaven both lied with Ansh. It didn't matter whether she was is heaven or not as long as Ansh wasn't by her side. So she kept on crying in front of the Gods that she couldn't just leave.
And when Ansh reached there, Lord Vishnu asked him whether he wanted to join Piya or not. But Ansh replied that he was guilty of doubting Piya and for that one mistake of his, Piya had been suffering all along. So he wanted to take Piya with him and repent by giving her all the happiness of the world. Joining Piya won't absolve him of his crimes.
So seeing their love for each other, Lord Vishnu & Mahadev let them go and told them that after everything they both have gone through, the Lords expected them to value each other from that moment on and never to get separated again.