The same shape, three different problems
A part that's easy to make one way can be difficult or absurdly expensive another way, because 3D printing, CNC machining, and injection molding build shapes through fundamentally different physical processes. 3D printing builds a part layer by layer from the bottom up, which means it handles complex internal shapes and organic curves well, but unsupported overhangs can sag or fail without added support material, and layer lines create some directional weakness in the finished part. CNC machining removes material from a solid block with a rotating cutting tool, which produces strong, accurate parts but struggles with deep internal pockets, sharp interior corners, and features a cutting tool physically cannot reach.
Injection molding forces molten plastic into a metal mold cavity and is built for producing large quantities of an identical part cheaply once the mold itself is made, but the part has to be designed so it can actually be pulled out of the mold afterward, which usually means avoiding certain internal features and adding a slight taper, called draft, to vertical walls. A design that works perfectly for a one-off 3D print can be impossible to mold, and a shape that machines cleanly can require redesigning entirely before it will injection mold.