I feel like this is a class thing rather than a regional thing. As a native New Englander who's spent his whole life eating strawberry rhubarb pie in season, relying on my grandmother's recipe, I too have been totally dumbstruck by people who've never had it and in my experience those people have generally been rich/upper middle class, people who've grown up with the money to afford more cosmopolitan pallettes. But that's mainly anecdotal, I could be wrong though. Apparently it also originated in Germany and the UK so there could be an ethnic component to it as well. It's one of those things whose recipes have been passed down in families for generations but it hasn't become a recognizable, marketable staple like apple or cherry pie.