Because you would compromise the greatness of what they have now with HP. Right now we have a near-complete representation of the most important area in the series: Hogwarts and its surroundings. The last 195 pages of Deathly Hallows takes place there! I feel that, for fans, the main demographic for WWoHP, lumping everything in the Wizarding world together would just feel fake and cheap. WWoHP gained such big press and attendence because it got out how well they made you feel like you were in another world. No matter how they do it, there's no way they can recreate what they have already done, and it would not be nearly as successful. If they wanted to make a Harry Potter expo, or a Harry Potter film set, then they would have done that. But they made a Harry Potter world, and in a Harry Potter world, Hogsmeade is just not next to Diagon Alley. YES, they could do it, it would attract an initial following, but the fans would be disappointed. Now the only thing I see for a fan-pleasing expansion is a lengthening of Hogsmeade or an addition of other landmarks with the Hogwarts/Hogsmeade area. Something like that would retain the contingency they, I mean Universal Creative, have worked so hard to make here.