But cancer doesn't immediately and permanently solve your relationship problems. And Kitty and Robert had major ones. Granted, they were related to Robert's focus on work versus home and Kitty's health crisis would lead to Robert having more of a focus on home, therefore making it seem like he had changed, but realistically, it would be more of the same after she went into remission.
And if I thought that the writers were thinking along those lines, or if we had seen Robert realize he did have a problem and resolve to change instead of him making a speech about it, I'd be okay with it. But Robert just made the speech and big change of dropping his campaign without going through the work (or us seeing the work), so it's seems more like a quick fix for the writers to get out of the corner they'd written themselves into with the relationship issues.
(Not sure if your transplant-remission-quickly comment was about my last sentence, but I meant that the writers seemed to have wrapped up the relationship problems, running for governor, and cancer storylines in one episode at the same time, not that the remission was unrealistic.)
An interesting real-life note to the cancer-marriage link was this recently published study
showing that women with cancer have divorce rates twice as high as the general population and six times as high as men with cancer.