The designers know what the people like, and the heads want to build what the money-spending people (in this case, HP fans) will like, and Diagon Alley right next to Hogsmeade won't cut it.
You're automaticaly assuming that an expansion WILL BE Diagon Alley. That, I doubt. It would take way too much work to get Jo to approve. Tell me, if the expanson was anything else would you support it?