It's the "Attack of Conventional Wisdom" syndrome.
"Conventional Wisdom" is that Republicans are stronger on defense and security issues than Democrats. Since we're at war, the only way Democrats can win, according to this simplistic view, is to be strong on defense issues. Having been a soldier automatically gives one the "gravitas" to put the Republicans on the defensive.
It's a symptom of the news-bite reporting of issues in the US these days. If something can't be reduced to 2-5 words (e.g., gay marriage, death tax, war on terror, family values, illegal immigration, amnesty for illegal immigrants, etc., etc.), it doesn't get covered even tersely in the popular press.
It's so much easier for reporters and editors to make a superficial presentation of the issues than take the time to get into the nitty-gritty features of the issues. And since, at least in the US, TV news often doesn't devote more than a couple of sentences to a story (gotta leave time for the ads and the promos for the other shows on the network), even if reporters want to cover more than sound bites the systems they work in won't permit it.
Why not obsess about their financial acuity, or their world historical knowledge?
Because conventional wisdom is that national security issues matter more than anything else right now.
HTH.
Cheers,
Scott.