For many, being able to stay in the park for extra time without the general public is THE biggest reason to stay exclusively on Disney property. I have not stayed off of Disney property for many years.
There's been speculation as to why and I think the Orlando Sentinel may have some pretty good insights.
"Investors have been pressuring Walt Disney Co.managers to boost profit margins in their theme parks. Closing parks an hour earlier some nights would help shave operating costs."
The Sentinel went onto say that Disney may want to reduce hotel-only hours to increase regular hours and boost capacity for all guests. Perhaps this has to do with the opening of much of their Fantasyland expansion in the Magic Kingdom on December 6th, which should draw increased crowds.
The Walt Disney Company said it has not decided whether the change will be permanent, although if you visit their site it explains their evening Extra Magic Hours as 2 hours not 3.