What were the main reason why John Kerry lost?

[RRusso1982]

In reality, it is very difficult to defeat an incumbent, especially during wartime.  By the fall of 2004, Bush's approval numbers were not great, but they were not horrible, like they became in his second term.  His approval ratings during the fall of 2004 were just below 50%, around 48% or 49%, and his disapproval numbers were around 46% or 47%.  Basically he was treading water.  The Iraq war wasn't that popular at that time but it had not yet become as unpopular as it later became. The economy was not great but it was recovering from the early 2000s recession.  Basically the election was a tie.  In baseball a tie goes to the runner.  In elections, a tie typically goes to the incumbent, unless the opponent is a really great candidate and runs a great campaign.  That was definitely not the case with John Kerry.  Not enough to justify throwing out the incumbent.  So the dynamics favored Bush narrowly.  Even so, Kerry still came one large state (Ohio) from winning.

[RRusso1982]

Kerry and the Democrats made a critical error.  By the fall of 2004, it was clear that the intelligence that was used as a justification for the Iraq war was bad.  The Democrats could have used that error and a better candidate to beat Bush.  The mistake they made was going too far in their criticism of Bush.  They accused him of knowing there were no WMD's in Iraq and deliberately falsifying the intelligence.  Kerry seemed to dial it back and say, "He misled us into war," and allow other Democrats to take it further and say, "Bush lied, people died." The problem with this argument, and Bush knew this, was that the intelligence that was used to justify the war in Iraq said the exact same thing that the intelligence during the Clinton years said.  So Kerry was put in a position of undermining a war he originally supported when the intelligence that was used as justification for that war was consistent with the intelligence that they were getting long before Bush was President.