Blood vessels have "compliance": that is, they are stretchy. Mathematically, this is very similar to capacitance in electricity.
The further from the heart you go into the vascular tree, the more the oscillations are dampened and the less it makes sense to think of systolic and diastolic pressures. In the capillaries and veins, blood flow and pressure is more or less constant rather than oscillatory (except in the most proximal veins where the motion of the heart again adds some pulsatile influences).
Therefore, for those vessels, you can just stick to mean values for pressure, the oscillations around those values are small.
See another Q&A: Why do people perceive blood pressure as the force that moves the blood forwards (see details)? that has a diagram showing the dampening of oscillations in pressure.