Simply put: when a 2D original's transition to a 3D turns out to be difficult, usually because a Video Game 3D Leap often requires more skill and talent than some developers have. There are two aspects of dimensionality when it comes to games. A game can be rendered in 2D or 3D, and the gameplay can be 2D or 3D. All 4 combinations have been seen. Switching from one of these combinations to another, especially going from 2D/2D to 3D/3D is fraught with peril. Particularly in the early days of 3D rendering, art styles that were painstakingly developed in 2D could be lost in the transition to 3D rendering due to lack of hardware capable of bringing it to life in 3D. And of course, there are some art styles that simply don't work in 3 dimensions at all.