When DISNEYLAND began spreading out around the Rivers of America, Walt Disney had already laid out the perfect location for a Haunted Mansion- just around the riverbend. It was perfect, except for one thing; Walt had no idea what to put inside it. He felt he couldn't leave that spot empty so the house was built despite having nothing inside it. DISNEYLAND did put up a sign hinting at what might be coming in the future for interested park guests.
Since the house stood empty with no real explanation as to why, park guests made up their own excuses. One urban legend said that the original house was so scary that a guest died of a heart attack and Walt ordered the mansion closed to be made less frightening. The truth- that the attraction just wasn't ready- was less interesting.