Because even after the event starts a house may not be available for a myriad of reasons, so they won't guarantee which houses you will see on each tour. That doesn't mean that house A, B, C are not set for the three-house tour, and houses D, E, F, G, H, I are not set for the six-house tour, they just don't want there to be a situation where for some reason house F is not available and they have to substitute house B instead, but because they stated that house F was part of the 6 house tour but you went in house B, you complain and demand your money back. (Because, like, never in the history of theme parks have people thrown temper tantrums at guest services.)