Part of being a big business like Square Enix has become is merchandising. From their days of being RPG-driven to increasing their stable with all kinds of shooters, adventure games, and other things, Square turned the Kingdom Hearts franchise from a PlayStation 2 exclusive into a multiplatform grab that can be found on everything from a Nintendo handheld over to your mobile phone. Disney has such a wide appeal, it's little surprise Square Enix felt the need to keep the franchise in one plot of land.
1. Kingdom Hearts has a long and complex storyline. Multiple games and side-games seem necessary.
2. Multiple platforms help sales while getting a more variety of gamers to play. If you do not like multiplatforms, get the HD Collection.
3. Kingdom Hearts will be big. Really big. And considering that many Disney places are already taken and used, more creativity is necessary.