Because so much work is done for you - PBR shaders have far fewer inputs. For example, if you wanted to have correct energy conservation with your specular highlights with a shader that didn't have energy conserving specular - you'd have to do it by hand in your Specular Level and Glossiness textures. Now it's just done…
You still can use them, and you should. Just, not baked into the diffuse. Toolbag 2 for example has slots for AO. In direct lighting, there shouldn't be any AO. AO is reserved for ambient light, not direct.