The butterfly effect.
No, it was not Kark Rove's "evil" brilliance, although it was important.

The election was lost because ultra-liberal interventionist judges in Kerry's own MA decided to promulgate a ruling that gay marriage, not just civil unions but marriage in full name was a right of gay & lesbian folk. No compromise. Now this ruling woulda been moot had a previous referendum defining marriage passed ba a large MAJORITY in MA been allowed to take effect but the (D) legislature ignored it, as the elitist betters done many timnes before. So, MA became the lightning rod for the specious (in my opinion) gay marriage issue. Turns out that several 'swing states' initiated referenda on the issue in reaction. This galvanized the milllions of religious right voters who had by and large sat out the 2000 election. Not this time. They voted. In droves. And not for the equivocator. So, Kerry lost, not because of a referendum on Iraq, on terrorism, or on jobs and the economy but because nearly 4 million religious folk decided NOT to sit this one out despite their prior 2000 disillusionmemt with Bush.
The long term historians will write it this way. You get the galley copy.