Incumbents tend to have an advantage in elections. People fear change and keep voting for the same people in power, which is why term limits are so important. John Kerry wasn't the most charismatic candidate to field against Bush either. He wasn't great at communicating his stances or demonstrating consistency, so people turned to the incumbent.
People back then were still deluding themselves that the US was doing good work in the Middle East. 9/11 was more recent to 2004 than the start of COVID is to us today, and so a lot of voters wanted to keep it going until it yielded the results they wanted.

[–] [email protected] 2 points 9 months ago

I'll add to that George W. Bush squeaked by in 2004 thanks to the incumbent vote, running against John Kerry, the most boring neoliberal candidate the Democrats had to offer, and Bush's campaign had to invent swift boating and utilize it against Kerry (showing the GOP will gladly throw veterans to the elements for power) to gain Bush's second term.