Main reason is you are using a 50/50 mix,which means water and antifreeze. Just because you have antifreeze in a system does not mean you have total corrosion protection. Water will make rust and scale eventually and as the 2 mix will start to also get somewhat acidic. That combination will also get more muddy with time and cycling of the motor. Cycling is the motor getting up to heat and then cooling down after it's turned off.