Your scene contains meshes with PBRMaterials. (Introduction to Physically Based Rendering | Babylon.js Documentation) These receive light from the environment texture.
You’re creating one in your code:
SkyMaterial doesn’t create an environment texture so it doesn’t light the meshes, this why you need to add another light source, for example the HemisphericLight.
If you add a mesh with StandardMaterial to the PG with the environment texture it will be not lit:
If you change it’s material to PBRMaterial it will start to receieve lights from the environment texture.
Please read the documentation for your own good to master lights, materials and all the stuff around it 
