I think size has a lot to do with it.
Paper is convenient for smaller prints, and a paper surface is better at small sizes where details in a photo would be overwhelmed by the texture of canvas.
At large sizes, paper is increasingly difficult to handle. Framing, matting and quality glass/acrylic become incredibly expensive, while the framed prints become more fragile and more difficult to handle. A 20x60" paper panorama is no fun to handle, let alone a 32x96" panorama - common sizes in canvas and metal. Canvas and metal prints (whether paper bonded to metal, UV prints or dye-sub prints) become much more practical at large sizes.