Aunt Alexandra tells Scout that she cannot have Walter Cunningham over because Aunt Alexandra is a snob. She is a strong proponent of the southern society's strict social class divisions. She is a Finch, and the Finch family is one of the oldest and most well respected families in all of Maycomb county. Aunt Alexandra thinks that fact alone gives her and the family a huge sense of entitlement. Walter Cunningham is not from a well respected, deep roots family. His family is kind and honest, but ultimately poor. For that reason, Aunt Alexandra tells Scout to be cordial to Walter, but minimize any contact she has with him. Aunt Alexandra believes that the Finch family is too good to be associated with the Cunngingham family. Aunt Alexandra tries to be a bit kind in her first explanation.
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.
But Scout pushes the issue, and Aunt Alexandra is then incredibly blunt and cruel in her reasoning.
Because--he--is--trash, that's why you can't play with him. I'll not have you around him, picking up his habits and learning Lord-knows-what.