Because in my thought on the configuration, in order for it to be the size and scope that is predicted it'll take up the entire back wall. This means everything from the current M&Gs to RRCTS would go. Also I predict they'd have to raze the entire land in order to enclose it properly basically making one big show building.
Here's 1.85 acres taking up the back wall...
Just a note: The 1.85 acres comes from reducing the 2 acres that TGMR currently takes up. Don't know where people are talking about using less space than that. That's the best guest for an efficient use of space with no extra space for M&Gs or anything else... unless you build another level.