The mistake comes from the understanding that Harris is a "good choice by dems" thinking. Nobody believes that, well not a lot of people at least. The idea is this; there is very limited time left, Biden "can't" be a president anymore, his health doesn't allow him, which makes you or I or anyone a better suited president, because we at least have our cognitive health. Wheras, Harris may not be "liked", she at least can speak clearly, literally the minimum requirement. So that makes her a better candidate from Biden already, not because she is "better", but because Biden is literally incapable. Secondly, if we were to talk about why not someone else? Well Harris already put her hat in the ring, whereas others are seeing this as a loss cause, which means that, if they try to be a candidate, or even become one, they will most likely lose. So why not pick someone like Harris, who everyone expects to lose, on an unwinnable election? So that you can pick someone better, like Whitmar, or Newsom, on an election that they may win?