Good question, with several answers. For one thing, I find Spinner's End a pretty depressing place, not somewhere I'd want the next scene set. For another, I can imagine Severus having somewhere else that he prefers to live when Voldemort/the other Death Eaters aren't around, somewhere he wouldn't want them to know about. Mostly though, when I ignore the last one or two books, I tend to ignore the backstory revealed in them as well. It allows me more freedom as an author, and means I can surprise my readers more easily. This particular setting is very specific in my mind because I've been writing it in another fic--the one the reality is based on.