Admittedly, she did become engaged to Rushworth before meeting the charming Mr. Crawford, but after being so unceremoniously dumped by Henry, I'm not surprised in the least that she stuck by her engagement. I'm not saying that her choice is any the better because of that, but I assume that his wealth was one of the reasons - OK, the main reason - she intially found him attractive. If she stays at home, her life and her prospects for moving away from that life will be much as they were at the beginning of the story, so I can see how marrying someone even as dull as Mr. Rushworth might be quite alluring.