The answer to 'why is my shower rusty water' is that it is likely due to corroded or old plumbing pipes in your home. Over time, the metal pipes can rust and release reddish-brown particles into the water, causing it to appear rusty. This is a common issue in older homes with galvanized steel or cast iron pipes.
Rusty water can also be a sign of a more serious problem, such as a leak in the plumbing system or a problem with the water heater. If the issue is limited to the shower, it may be isolated to the showerhead or piping in that specific area. However, if the rusty water is present in multiple fixtures throughout your home, it's likely a more widespread problem with the main water supply lines.