Because Joe Biden was too old and shouldn’t have started to run for President again and because Kamala Harris was a bad candidate? Or do you mean something else? You can complain about Trump, but the people had a choice.
Trump is both of the above, too old and a bad candidate - as you can see from the current state of our country.
You don't have to assume they're stupid, it's just that they're entirely surrounded by a bubble of increasingly protofascist craziness framed as reasonable opinion. Turn on Fox News sometime, or open Twitter.
If you allow yourself to live in such a bubble and don't seek alt knowledge, you're stupid.
How would you know you lived in such a bubble?
For example,. talking about captialism being great and communism being evil and then instituting worldwide random and massive tariffs, promising to start no wars and then starting multiple wars of pure choice, believing in free speech the getting arrested over jokes about a persons death are objective facts that can be observed from any bubble. If one chooses to ignore it or claims it doesn't exist, then one is stuck in denial. If one can recognize this and step out of thr mess, then one succeeded in recognizing and escaping the bubble.