Couple reasons I imagine. Metal/Rough are both gray scale images in PBR. Spec/Gloss in a PBR workflow requires that Spec be RGB. is because it’s Diffuse texture only holds the color information for non-metals and is black where it’s metal. The Specular texture is black everywhere except where the metal is, then displays the color of the metal where there is metal. That is two extra channels, and more confusion.
The other reason is Base Color/Albedo. Metal/Rough enforces a strictly PBR safe color texture. Spec/Gloss as a shader is still somewhat behind, even as a PBR workflow, if only because it relies upon the old Diffuse shader.