Lots of reasons. Left-leaning Canadian’s take:

• Biden/Kamala have acted against the American majority for months now in Gaza. This isn’t a sabotage take, it’s as close to fact as can be determined of national attitude. Look at my post history before you accuse me of “both sides”. A lot of Dems and especially young people are very vocal about hating support of the ethnic cleansing of Occupied Palestinian Territory. Millions of people chose to stomach it until the election, but I think other millions felt unrepresented and betrayed by protest suppression and bipartisan condemnation.

• I don’t want to insult anyone, but even moderately detailed political plans may not work in America. Trump’s campaign was run on vague promises and angry rhetoric that was emotionally engaging. Clearly people don’t know how much Trump’s policies like tariffs are going to hurt them personally, but like promises of a strong “America for Americans”. Kamala ran on a detailed platform that took effort to understand and clearly it failed to motivate enough voters.

• “Try to please everyone and you will please no one.” Instead of solidifying support and inspiring hope among the Dem base, Kamala’s campaign assumed their support due to fear of Trump and went after undecideds and Republicans. Republicans aren’t going to switch and many undecideds are that way due to apathy.

• No Dem primary meant people couldn’t choose the candidate they might actually want to support. They were given Biden then Harris without being asked for input.

• Related to that: Biden’s campaign soured voters, and Kamala wasn’t able to climb out of the hole left to her. His low approval rating didn’t help given Kamala felt like a younger version of more of the same.

• Misinformation and propaganda by foreign and domestic right-wingers kept a lot of people from switching sides.