People will cite a lot of factors for this. Inflation, unwanted identity politics, not being able to string a sentence together, etc. None of them are incorrect.
But what really kicked off this decline was Afghanistan. He was at 50%+ approval before that, and hoped he would be able to weather the media storm for a couple weeks, he was wrong.
That type of cowardice, abandonment and disregard for human life is completely antithetical to American's national self-image.