Why do you think Mrs. Freeman and Mrs. Hopewell are introduced before Hulga?
By introducing the two older women first in the story, the author allows us an opportunity to understand Mrs. Hopewell's desire for her daughter Joy/Hulga to have a normal happy life. Mrs. Freeman's daughters, Glynese has many admirers, Carramae got pregnant at 15 and is married. Both her daughters are on the path to the future that both these women want for their daughters, husband, children, home of their own.
Mrs. Hopewell must listen to Mrs. Freeman talk about her daughters knowing that her daughter is bitter, distant, and alone. So it gives the reader an understanding of why she is so eager to invite Manley Pointer into her home.
When she sees the Bible salesman, she believes that maybe he could be a suitor for Hulga. Mrs. Hopewell, so named for her eternal hope in life and its prospects, that she looks at this man with her eternal hope and thinks maybe she can get the desired happiness from Hulga afterall. She longs to be like Mrs. Freeman, also so named for her ability to be free from the worry of having to marry off her daughters, they are both either set or nearly there.
It is probably to establish a clear contrast between the two ladies and Hulga. Mrs. Freeman and Mrs. Hopewell are good, God-fearing, country people. They are content to accept things as they are, and do not question their faith or way of life. On the other hand, however, Hulga is an athiest free thinker, which is frowned upon in this community. By introducing Mrs. Freeman and Mrs. Hopewell first, O'Connor sets up a wonderful view of good, "country" people, only to disrupt this harmonious way of life with one who seeks to undermine it through free thinking and outside, liberal influence. However, as fate would have it, she is sucked into losing her leg by Manly Pointer, the "good, country" Bible salesman who truly does not believe in God. Or, as he puts it, he's been "believin' in nothing" for as long as he can remember.