BMWs in general are known for the plastic coolant parts failing somewhere around 60-100k miles. Maybe even faster if you live in a warm climate. It is the nature of plastic and it is a service item.
You don't expect BMW to cover all of your radiator hoses and belts do you? Those wear out just the same. After 8 or so years plastic that is constantly getting hot and then cold, hot and then cold will get brittle.
As to why it was not replaced when the other work was done.. Not sure. People usually want the cheapest way to get in and out of the repair shop. Most people seem to wait for these parts to break before fixing them. |