because is renders a larger world then crysis.
and the art style makes it look like it shouldn't be as demanding as it is.
Xray engine 1.5 (clearsky version) actually is a more advanced engine then crysis in many ways.
Advancements made in 1.5 include volumetric light, dynamic volumetric smoke, full volumetric fire, soft water, dynamic wet surfaces (with water streaming down the sides of surfaces), depth-of-field blur, DirectX 10 support, SSAO (Screen Space Ambient Occlusion). A complete day and night system is included along with improved weather effects.
SSAO is the main difference, its extremely demanding