I actually prefer Treasure Planet to Treasure Island. A pirate story set in space is a cool idea. It's a very nostalgic film for me because I used to watch it a lot when I was younger. It perfectly combines futuristic and retro elements. I am glad that a sequel wasn't made because usually sequels are not as good as the originals.
I don't understand why it was one of Disney's greatest faliures. I loved it. It's not my favorite but it's a GREAT movie for family and for you to watch in your free time. I think it deserved more than it got. They were going to make a sequel at the time but because it did so badly at the box office, they scrapped all of the ideas. I think it was worthy of a sequel and is a lot better than people make it out to be!