The game keeps it's 2D structure primarily to deter users from feeling intimidated. It's much easier to work with a 2D world, anyway. The intense physics engine helps keep things fresh and prevents the game from becoming just a 2D platformer. Keeping things approachable and keeping the single-player experience fun and satisfying are also keys to success. While some levels can be designed with teamwork in mind, if it's not fun alone, it probably won't work online.