The answer to 'why is my supply line low water pressure' is that there are several common causes, including a partially closed shutoff valve, mineral buildup, leaks, or a problem with the water main. Low supply line pressure can lead to reduced water flow and poor water pressure throughout your home.
First, check the shutoff valve on the supply line to ensure it is fully open. Over time, valves can become partially closed, restricting water flow. If the valve is open, the issue may be mineral or sediment buildup inside the supply line, which can narrow the pipe and reduce pressure. You may be able to clear this buildup yourself by flushing the line, but if the problem persists, you may need to replace the supply line or call a professional plumber.