Kingdom Hearts has always been visually appealing and for a game from that time period to have great graphics that grab your attention was unheard of. You also had the level of detail that was put into each of the worlds that you played in. Many of the reviewers mentioned how impressive it was.
The main reason that Kingdom Hearts has not blown away the sale numbers that come in for The Witcher or Fallout is because the game is appealing to a very different demographic. One that is younger, much younger and one that even though more children are playing games these days, it is not likely to help this franchise now.
Fans are hoping to login into their favorite console with the game loaded and see a world they remember from a movie. Disney could make so many more and the buzz behind what worlds are going to be added is likely to be what is keeping the game alive.