Why is blood pressure higher in left ventricle than in right ventricle?

The pressure in the left ventricle is significantly higher than the pressure in the right ventricle. This is true with the assumption that you are referring to ventricular systole (contraction of the ventricle). The left ventricle forces blood out of the heart into the aorta to all parts of the body, and that same pressure forces blood back through the veins.