Why does Aunt Alexandra say that Walter isn't their kind of folks?

From the text:

“Don’t be silly, Jean Louise,” said Aunt Alexandra. “The thing is, you can scrub Walter Cunningham till he shines, you can put him in shoes and a new suit, but he’ll never be like Jem. Besides, there’s a drinking streak in that family a mile wide. Finch women aren’t interested in that sort of people.”