Based on her comments regarding the Hill/Giler treatment, it sounds to me almost like, with the Blomkamp one crashing and burning, she just doesn't have the strong desire to return to this character in general now, regardless of the creative force behind it.
Perhaps, with her so neck-deep in Avatar sequels at the moment and Jimbo briefly entertaining the idea of producing Blomkamp's outing, there might be a bit of Cameron loyalty in there too; but that is honestly just baseless speculation on my part.