Modern graphics hardware isn't optimized for drawing bitmapped lines. It's optimized for drawing textured triangles. This means that it's not actually a good fi

Modern graphics hardware isn't optimized for drawing bitmapped lines. It's optimized for drawing textured triangles. This means that it's not actually a good fit for GDI any more. Given that Windows Phone was, as far as apps were concerned, a clean slate, there was little reason to retain USER and GDI. win32k.sys is also rife with security flaws, so scrapping it has arguably reliability benefits too.