Well the simple answer is because being "tough" is a major part of the Republican platform, and one which they talk about a LOT. They are also marketing themselves towards the segment of the population more likely to have a stereotypical (and if you ask me, superficial) view of "tough". And for that matter, they market towards the segment of the population more likely to focus on distinctions like "tough" and "wussy".
For that part of the population (which might, at best, be a slime majority), how you act and how you talk is much more important than what you do. Because to them toughness and often by association, masculinity, are things you need to prove to everyone around you, how you present your actions is much more important than what those actions are. This is why Bush got tagged as a tough guy compared to Kerry's pansy-ass, nancy boy, in the 2004 Vietnam debate. For the folks who cared about such things, it didn't matter that Kerry actually fought in the war while Bush actively avoided it, Bush talked the talk and was pro-war, while Kerry mostly did not and was anti-war. Yeah, it's as superficial a judgement as deciding someone is a manly man because he drives a huge truck with a "terrorist hunting permit" on it...but who do you think is the target audience for such debates anyways?
Really the premise of the question is wrong, because it's a one-sided debate. It's not that there aren't people who think the Democrats are tough and strong on national security, it's that the people most likely to hold those opinions don't think about the debate in those terms.