Mercedes and Edmond don't get together because neither one feels love for the other any more. At first, Edmond is angry at Mercedes because he believes that she betrayed him by marrying another man. After a while, he forgives her enough to leave her the house and small sum of money, but he can never love her again. When they were young, he was a naive and idealistic boy who idolized Mercedes. Both of them have changed - he's now incredibly cynical, and Mercedes isn't a good idol because she's tainted by having aged and gone through tragedies. On the other hand, Mercedes can't love Edmond because he caused her husband's death and dishonored her and Albert.
I think Edmond and Haydee do form a romantic relationship at the end. The first time I read the book, I was confused because Edmond brags that he has a slave instead of a mistress. But after the second time I think that comment was just one of his many exaggerations, and he and Haydee live strictly as a father and daughter until the end, when they realize that they love each other romantically too.