Objectively, there's no basis for this. As Taranto's readers have pointed out, the military is full of high achievers, not "losers." Our troops are overall better educated and more intelligent than the general population in their age groups. Yet, this feature of the liberal mindset is very hard to shake, and it's worth asking why that is the case.
To avoid being accused of "questioning their patriotism," I'll speak of liberals in general, rather than Kerry and Rangel in particular. There is a strong current in liberal ideology that holds that patriotism/nationalism is, at best, naive. This is a popular sentiment among European leftists, and it is voiced more openly over there. Patriotism, particularly American patriotism, is seen as evidence of a lack of sophistication. In this view, feeling patriotic is only for rubes who are being manipulated (by conservatives, of course). The idea that something greater than oneself might actually be worth fighting for, and risk dying for, is "obviously" wrong, and no one in the know seriously believes it. Since our troops have signed up to do that, it follows they are either foolish, lacking any good alternative, or both.
This also explains the prevalence of the "chicken hawk" attack meme. Whether or not someone personally served in the military has little bearing on the validity of his opinion on matters of war and peace, of course. However, to an anti-war type who believes implicitly that patriotism and military service are only for dummies, lack of military service in a "hawk" is a gotcha. The liberal projects his own disdain for patriotism and the military onto the hawk. The hawk must also recognize that it's all a cynical game, since he didn't fall for the scam.
Liberals also view conservatives as dim-witted. Since the military is much more conservative than the general population, that also fits the "troops are stupid" theme. So mere facts and statistics are not going to persuade the liberals otherwise. They don't even realize how condescending and obnoxious this attitude is.