From Scientific American —
Why is normal blood pressure less than 120 over 80? And why don’t these numbers change according to a person’s height?
But modern epidemiologic studies have confirmed with a great deal of certainty that risk of a heart attack or stroke begins to increase in adults when the usual systolic blood pressure is 115 or higher and/or the usual diastolic blood pressure is 75 or higher. The risk steadily increases with higher and higher readings, so the traditional 120/80 level remains reasonable as a threshold for getting a doctor’s attention.
It does in fact increase with height, which ensures that the brain, located at the highest point of the circulatory system for most of the day, gets sufficient blood flow and oxygen, despite the pull of gravity and other forces. But the effect is fairly small, which is why the 120/80 figure is not adjusted for taller people.
Answers excerpted from fuller explanation.