I think it's just that life changes so much that the fiance just doesn't want to be a part of it, though I think it reflects more on the nature of the relationship. Marriages rarely end after SCI, and you'd be amazed how severely mentally injured a person can be and have their spouse just double down and dedicate their lives to ensuring their spouse can live at home, to include taking out second mortgage to change every door frame width in the home, and make the kitchen and bathroom full accessible. In those situations you can't sense even a second of hesitation, and you get the strong impression that the spouse is stronger than the patient.

With the fiance, they're imagining their fairy-tale wedding, their fairy-tale honeymoon, and those wonderful years before children enter the picture.   It's sharing a life and having no limits.  This is half of a relationship, the other half being really and truly getting to know another person, to be vulnerable, and exposed, and to still love each other no matter how much dirt you dig up about the other's psyche.  When the focus is on #1 (the physical life together), the relationship ends.  When the focus is on #2 (psychological intimacy), nothing can stop it.  I think the reason the fiance gives up on the patient, is that #2 wasn't formed very deeply, and the fiance doesn't see the accident as what happened to the patient, but instead of what just happened to themselves.  

What bothers people about losing #1 would additional concepts about being a provider, a defender, or fulfilling traditional gender roles.  In this case, some women are very happy to live with a man who can check out bumps in the night, and after an accident, she's now the one to take that responsibility.  Things also change with basic bowel and bladder habits, with some people now having to self-cath, or have a needfully elaborate bowel ritual to ensure their body is vacating regularly.  Then there's the worst topic, and that is sex is completely changed.  Young people especially have difficulty adjusting to this concept, as in your 20's and 30's sex seems to be a focal point of relationships.  Nothing is the same.  This world is not wheelchair accessible, and at least to me, everywhere fun has either sand or a dirt trail leading to it.  

Everything changes, everything is different, and the fiance doesn't want that to be their new normal. In the most unfairness statement of them all, the patient doesn't have a choice about their new life, and it's the source of their positivity and resilience. The fiance does have a choice, and unless there's a strong and loving relationship, few people choose to be part of that life.