Because the STEP file has higher fidelity and can be edited as more or less common primitives. The 3MF file is just a bunch of triangles wrapped in print settin

Because the STEP file has higher fidelity and can be edited as more or less common primitives. The 3MF file is just a bunch of triangles wrapped in print settings (it’s literally a ZIP with the STL inside, which is, in itself, the aforementioned bunch of triangles). Shapr3D can import and export pretty usable STEP files.