Walt Disney did not want to give his guests the impression that his park was run down, so he wanted Imagineers to take care of the outside, while the ghosts take care of the inside. However, the Tokyo Disneyland Haunted Mansion and the Disneyland Paris Phantom Manor have neglected exteriors with overgrown gardens and broken windows.When Walt and his imagineers were creating and developing The Haunted Mansion. One of Ken Anderson's 1955 sketch of a planned haunted-house attraction was supposed to be dirty, decaying, and dilapidated. It was originally to be along side Main Street USA. But with many plans and time; the technology at that time was still in development, so it wasn't made yet.
In order to keep true to his park's cleanliness, he wanted the mansion to like clean, new and well taken care of. He didn't want a run-down dirty broken old mansion at his park. So, his old saying was this: "While we take care of the outside, we let the ghost take care of the inside." He didn't want a bad impression of a run-down park like of those old, so he had his imagineers to take care on the outside of the park. His philosophy on the classic attraction is true to this day. The mansion outside to this day looks neat, clean, welcoming, and always in good condition. The inside is the opposite, it is dusty, dirty, filled with cobwebs, and filled with old and broken things. Since, the mansion is a retired earthy plane for ghostly residents that enjoys the afterlife and instead of moving on, they enjoy staying earth bound and re-living their old live pasts.
Thanks for taking the time to answer my question. It clears up thing a lot. I've always wondered why it was like that.