Why didn't Carrie and Big get together after Natasha left Big?
To me, this makes no sense. They had been so attracted to each other that they risked and lost their primary relationships as a consequence of their affair. They had both said 'I love you' to the other.
What prevented them from getting together after their partners had left them?
All I can think of is the writers did not want to show their happiness based on causing others pain. But they had been with the wrong people. Big was going to leave Natasha anyway and in the end, Natasha left him. Carrie didn't dump Aiden; she wanted to stay with him, but he rejected her.
It's also possible Carrie realized that Big was a compulsive cheater who had hurt both his wives by his infidelity, which was the form his inability to commit took within marriage. She foresaw that he would only hurt her again.
But if so, why wasn't this explicitly stated?