It was written about in a couple places that the complete Falcon built for ESB was carefully dismantled, mothballed, and placed in storage so it could be used again for Jedi.
It's possible they only reassembled what was needed (or be visible) for the sandstorm scene.
And it's a damned shame nobody thought about giving it to a museum after that!
I'm curious as to why they didn't use it for the Rebel hangar scene though. The matte painting behind Han and Lando always sticks out for me.