I'll not stand for this slander. DI is the perfect tutorial level.
Lets you practice battling as much or as little as you want and allows you to learn about things like Tech points and Revenge Value in a controlled, no-stakes environment.
Gives you the option of taking on some marginally harder challenges that require more skill but not perfect execution, with rewards that are useful enough that it feels worthwhile but not so glorious that they're gamebreaking for those that get them or punishing to those that don't.
Encourages exploration early on by hiding the Protect Chain in that little nook for clever enough to figure out how to get it. Again, nice to have, but not super powerful.
Sets up the central friendship between Sora and his buddies, and it feels natural and believable even in its limited time. You even get to compete against Riku in a couple different methods if you like, where he's considerably tougher than anything else on the island but not unbeatable.
Quickly develops the sense of adventure and air of mystery that so defines the game.
Short and sweet - if you know what you're doing you can get through it in minutes if you like, and even if you don't know what you're doing it doesn't take long at all.
Has an incredibly pleasant soundtrack that captures the peaceful nature of the world while still being lively and fun.
Twilight Town in KH2 does some of that, but it's all just worse. It takes way longer no matter how familiar you are with it, a lot of it feels like tedious busy work, the music is somber and forlorn and sounds like it's heralding the end of an adventure rather than the beginning, you really have no idea what's going on for most of it, the world is bigger but there's less exploration and agency so it kind of feels wasted, etc.