I know that the soil was one of the reasons that the Horizons building had to be demolished, also it couldn't accomodate Mission: Space, but I'm not sure if that's the reason the ride was closed. It may be because the ride was closed for a very long time before they began the demolishion.
Other than that, I'm sure that Disney did some sort of study or analysis and realized that Horizons was not getting the attendence that they wanted (even though I LOVED that ride). Disney's motto is if its more than a few years old, it must be bad."