Maggie's an insensitive bitch who, at the first sign of real trouble, drops a relationship like a hot potato?
Seriously, though, Joe's behavior towards Maggie wouldn't exactly inspire much support. Yes, he says he loves her - but then almost everything else he does contradicts that from Maggie's POV, and she gets no substantial explanations whatsoever as to why that is. We as the audience know why, but Maggie doesn't. And with Joe there's never any generally known indication that he's been attacked and is suffering from a loss of blood as was explicitly the case with Maggie. Joe doesn't have a Sam in his life who insists that he's going to see a doctor whether he wants to or not. But then, that angle or even Maggie forcing her support on him wouldn't have really worked in this plot because one of the points would certainly seem to be for Joe's life to completely fall apart and for his and Maggie's relationship to disintegrate, paving the way for Nicholas to swoop in. If Maggie had been as supportive of Joe as Joe had been with her, chances are none of that would have happened.
The disintegration of Maggie and Joe's relationship is perhaps one of the biggest tragedies on DS.