There is no one simple explanation.
First, Kilgore was a mediocre candidate (I posted this before), so he neither galvanized his base nor attracted others outside his base.
Second, more importantly, in Virginia (as elsewhere across the nation) the left saw this year's election as a chance for revenge for their losing streak from 2002-2004. They turned out whereas conservatives were lackidaisical.
Third, the media played a major role in supressing stories harmful to Kaine. A case in point, Carlos H. Bustamente Medieta (an illegal immigrant) brutally mudered Hak Bon Kim (a legal immigrant) in northern Virginia just a few weeks ago. This story was quickly hushed up.
Fourth, Kaine sucessfully hid behind Warner. Warner was a very popular moderate Governor, whereas I believe time will tell that Kaine will end up being a very unpopular liberal Governor.
How do you like eating crow? Face it, you were dead wrong on Kilgore winning (even the percentages you predicted) and you were dead wrong on illegal immigration being any kind of an issue (it wasn't). So now, you blame the media! A classic Republican tactic, and yet you still call yourself a Democrat.