Unfortunately, military service is not a distinguishing mark of service within the current government. Unlike the US, where I think 30% of house reps and senators have a background in the armed forces, the numbers within Canada house is microscopic. I can not think of anyone within the current government who has served. Do you think McCallum could get past basic. While he was Minister his people tried to get him checked out to do a backseat ride in a Hornet as a PR stunt. Needless to say it didn't happen becasue he wasn't medically fit.
The view held with the current ruling party is that Defense is a secondary or even a third level cabinet post ranking with Transportation, Industry, and Fisheries. It is viewed as not as important as Foreign Affairs, of Finance.
The question is not if the Minister should have a background in Defence, but rather why is Defence viewed as not important.