Sorry but a Haunted Mansion overlay is not going to justify the cost in terms of bringing new guests to the property the way it would in Anaheim. It may pull guests from other areas of the resort but it's just not the kind of thing someone is going to plan a trip around. They know this. They've done much research.
The other thing which is a factor and part of their "official" line as to why they don't do these things: Your average guest saves up 3+ years for a trip to Walt Disney World. Many come only once or twice in their lifetime. When they come, Disney believes, they want to see The Haunted Mansion or Space Mountain, as intended. They'd be disappointed to see cheap holiday overlays from something they'd waited, essentially, their whole lives to see. This also doesn't account for, as others have said, the extended downtime for adding such overlays.