Geometry

Not being flat space is not a reason why representing 3D in 2D is difficult. In fact, 2D space is flat space, and the difficulty arises precisely because 3D objects are being represented in this flat, two-dimensional environment.

Representing 3D objects in 2D space is difficult because:

Perspective issues: 2D representations often rely on perspective techniques, which can distort the proportions or angles of the 3D object.