Professor: The Crusades were specific military actions during a specific time frame for specific reasons that have a specific name attached to them (hence the capitalization of the word - it's a proper noun). On the other hand, terrorism is a general term. Not all terrorism is committed by Muslims, and terrorism is not limited to a specific time frame; it has happened throughout history.
It is currently unknown what specific proper noun will be applied by future historians to the terrorism committed by Islamic extremists in the early 21st century.