'cause you can pick up the parody first very, very easily. If the parody is particularly funny, the chances that it's better known than the original are reasonable. And even so it can be just chance which comes to hand first. Or not. I seriously doubt that nowadays many people have read any of the Regency Gothics before they read Northanger Abbey.
It can be a very unsettling feeling, sitting down to a book and reading along and noticing -- oh, that's what Terry Pratchett was parodying in The Truth. (I read Neverwhere after The Truth.)
Or when you are reading something you know is a parody, but which you have no notion what the original was like. . . I was going through the Bob And George archives, and while many of the strips are funny without it, sometimes you just suspect that actually having played Megaman might make some of them funnier. . . .