Look at it this way, HBO/Universal would have had a specific announcement in mind. They would maybe release a video that showed specific scenes from the show that were coming to the house.
If it gets leaked early then HBO might be worried that fans would create their own video that the general public would see and assume is a representation of the house. Most of us in this forum know that unless Universal makes the announcement it isn't official. But Joe Blow on the street? He might not be so savvy, so he sees a fan made announcement that doesn't accurately represent what the house will look like, then gets peeved when the final product doesn't match his expectation.
HBO want to avoid this by controlling what the announcement looks like, and when it happens.
I'm not saying this is fact, just one possible reason why they might want to control the narrative to such a degree.