The principle reason is that Kerry does not talk much about it and nobody has been able to hang him with it.
His endlessly touted Vietnam service served ironically and justly to give traction to the Swift Boat Vets's effort against him.
Kerry is wise not to much raise the issue of his Senate record to a prominent point of his campaign. The paper trail of so many years is much more extensive and accurate than the combat records and citations of four months service in Vietnam 35 years ago. His laziness, non attendance, bloviations on the Senate floor, and steadfast opposition to so many Military programs and Anti-Soviet foreign policy positions will scream to high heaven if brought to the fore. In short, he is a liberal Senator from Massachusetts.
I've got an idea that may change in the debates, but it's tough to call him a non-entity AND maintain that he's voted against nearly every major weapons system in the past 20 years. the two ideas seem to conflict, and team bush seems to prefer the latter.