surface of the DTK 220HD vs my new Display Cintiq 24

why is wacom DTK 2200 HD i use at school that is like super old have this hard nice glass surface and my brand new Cintqu 24 inch i bought a week ago have this other type of surface with the pro pen 3 and i find doesnt draw as smooth ? Why did they change it? Did they still sell this older glass on newer model like the cintqu pro 27 or are they all changed