]Would it be, perhaps, because they wouldn't have to announce their banns and have tongues wagging weeks in advance of their marriage, but instead just present their community with a 'fait accompli'?

This was the impression I got - that if they went the normal route and posted banns, there would be too much opposition. He was not nearly good enough for her, let alone his age. His situation, his lack of money, etc. I expect her brother would have worked on her to abandon the idea. It's too bad he didn't, considering the rotter Shackleton turned out to be! I do wonder what made her do it. Her life got so much worse after she married him, poor thing...