Why do changes in preload not cause changes in end systolic pressure?
Preload is defined as the volume of blood that stretches the ventricle at the end of diastole. Therefore, when preload is increased, the end diastolic volume is increased. In simpler terms, there is more blood in the heart just prior to contraction. Even though there is more blood being ejected from the ventricle during systole (that is, there is an increased stroke volume), the increase in preload alone will not affect the end systole volume or pressure.

When preload is increased while afterload and inotropy are held constant, the cardiac muscle fibres will still shorten to the same length. This means that the end systolic volume is unchanged, therefore the end systolic pressure is unchanged. This is best visualized by a pressure-volume loop. On the first example on the diagram below, you can see that the end systolic pressure and volume at point 3 are unchanged with increased preload:



Since cardiac output is equal to the product of stroke volume and heart rate, with increased preload the cardiac output is increased due to the increase in stroke volume. The reason why the end systolic pressure is unchanged with the increased cardiac output is because the end diastolic volume is increased.