Because kidney function is damaged by pressure but its control mechanisms are governed by osmolarity (flow). A high pressure, low-flow kidney will send signals to the body to increase BP further, until the kidney essentially annihilates itself. You've got to unload the kidney down to a reasonable level, while keeping sufficient pressure to perfuse the heart and brain. In the setting of cardiovascular disease (stiff pipes with reduced cross-sectional area), the safe range becomes progressively narrower.