Animation is also a lot harder in 3D. A decent 3D engine will probably need fully featured skeletal animation: blending and interpolation, translate+scale+rotat

Animation is also a lot harder in 3D. A decent 3D engine will probably need fully featured skeletal animation: blending and interpolation, translate+scale+rotate, inverse kinematics, etc. A 2D sprite-based game might be able to get away with a much simpler animation system and still look great. E.g. sprite flip-books and really basic translation will get very far for a lot of game styles. Even if you want rotations, you probably don't even need to learn about quaternions since 2d rotations are so much simpler and you might not need to do interpolation/blending. A lot of other parts can be more complicated as well. E.g. testing navigability is difficult in 3D since it's not always clear what you are able to jump on to/fall off of/squeeze through. Relatively common bugs in 3D games are falling out the bottom of the map, escaping out the side, or getting stuck, but they almost never happen in 2D games. Controlling the camera requires a lot more thought in a 3D game. In a 2D game, a fixed camera or a camera that rigidly follows a character is often sufficient. In 3D, the field of view is awkwardly shaped in world-space, and obstructions are more likely and you may need a way to deal with them (e.g. manual camera controls, detecting obstructions and making them transparent, or moving the camera to avoid them). I guess a lot of this comes down to being able to design a 2d game that requires a very simple engine while even seemingly basic 3d games seem to need tons of features to be palatable.