Jo March's story takes place in New England where the morals and values of society are already established. Jo has the support of her family throughout as she tries to find her place in a society where women are not supposed to exist outside the home. Her independent spirit allows her to become a businesswoman, author, and a wife. Jo marries not because she's expected to by society, but because she finds a man who considers her an equal. Today, we would say they are "soul mates".
With a setting based in the West, Mattie Ross lives in a society of outlaws and nonconformity. There aren't always established rules for people to follow, especially as a woman. Mattie doesn't have the strong family support that Jo does either. When her father is killed, responsibility falls on her because her mother is ill, and her siblings are too young. Her belief in "an eye for an eye" requires great courage on her part, and she never backs down. Although she and Rooster are in love, Mattie knows she must go back and take care of her mother and her siblings. I think she remains unmarried because she never finds another man who has Rooster's "true grit", a characteristic Mattie would require of any man she married. Perhaps, Rooster is her "soul mate", and she is unable to find a man who could replace him. She does have a successful banking career, testifying to her independent spirit.