I think Kamala Harris lost because she was the default candidate after Joe Biden, influenced by age-related bad judgement, insisted on running for a second term.

But I also think US presidents pick bad vice presidents on purpose, and that both Kamala Harris and Joe Biden himself were picked because they were bad presidential candidates.

There are two reasons why you would pick a bad vice president: one, probably the major one, is that the president gets to appease both the vice president's followers (hey, at least we got the vice president!) and the other losing candidates followers to some degree (hey, at least that vice president is so bad, they won't be a shoo-in as the president's successor!)

The other reason is that they want to discourage impeachment and assassinations. You may think that's stupid, but as my rule #1 says, powerful people believe all the stupid things regular people believe. So even if it is stupid, if we have thought about it, they have thought about it too.