Honestly, I don't know. I think a lot of it has to do with the fragrance. Wearing clothing washed with a different detergent feels odd, at least to me. Also, one of the main odors that defines a "household scent" is the detergent used in that household. Maybe these factors help drive brand loyalty?

It's possible that this is a viral marketing campaign to get you to think exactly that. Other comments go into detail, but it seems impractical to do this on a large scale as mentioned in the article.