Politics is Political
Matt Stoller brings an important perspective to the issue of the media under-covering foreign policy issues in the campaign, namely that the liberal wonk community needs to share some of the blame for this since part of the problem derives from Democrats' reluctance to engage politically with these issues. That seems right to me. Any Democratic campaign worth its salt can provide for you a wonky discussion of its health care plan or its trade policy. But it can also generate TV spots, quips from surrogates, somewhat unfair mailers, etc. that don't at all sound like a seminar on trade policy.
Historically, they've been much worse at doing this on national security issues. A presidential campaign knows it needs to check the national security box, so they organize one or more Major Foreign Policy Addresses and then kind of play duck-and-cover hoping that Republicans won't attack them and when Republicans do attack them whining that you shouldn't play politics with national security. But if we all take for granted that politics will be played with basic questions of economic growth and fairness, then why not play it with national security, too? And beyond that, "ought" implies "can" and there's just no way to hermetically seal foreign policy off from politics -- one needs to learn how to play the game well. I even wrote an article about this once that became part of the backdrop for Heads in the Sand.
I do think things are changing on this front to some extent, though mostly in a sense of Democrats either getting smarter about playing defense or else the ones who are really bad at defense got killed off in 2002 and 2004. Most Democratic political operations still seem to me to primarily look at national security issues as an area where you might lose points, and not as an area in which to be on the lookout for potential lines of attack.