At the beginning of the book, it's clear as can be that Mary Alice is not excited about going to a small town and staying with her Grandma Dowdel for the year. So why does she go back to her grandmother's house to get married instead of staying in Chicago? Good question. Especially since, as Mary Alice tells us:
It would have been much easier to get married in Chicago. I'd held on to the apartment in Rogers Park and took the El every morning down to the Tribune Tower to my cub reporter job. (8.3)
Here's a good answer: Over the course of the year that Mary Alice spends with Grandma Dowdel, she comes to appreciate the small town lifestyle—and more than anything, to associate her grandmother with a sense of home and safety.
Of course, when it's time for the wedding, there's no one else around. Mary Alice's brother is fighting in the war and her parents are in Seattle. But we don't think that's the only reason the wedding is held in Grandma D's front room. Indeed, Mary Alice tells us herself that the location is important to her.
Though it meant I'd have to ride the wartime version of the Wabash Blue Bird, sitting on my luggage in the aisle, I wanted to be married in Grandma's house. (8.3)
Makes sense that the home where she made her last transition—from child to adult—should also be the place where she makes her next big life change: from single to married.