Mostly for Laurie's sake, really. I find it so hard to believe in him and Amy. I do think Jo and Laurie would have found it hard, but I think they could also have found it fun and worthwhile to do the hard thing and love each other anyway. And because I think Alcott sets you up for a book and a half to expect and want them to be together, and I love that part of the story. To tell you up front that it's not going to happen seems bizarre to me.