Just speaking for the Black voter portion of the drop off: There were literally surveys.
https://hachyderm.io/@mekkaokereke/113446192875604328
And Black folk, especially Black men 🙋🏿♂️have been *extremely* vocal online about exactly why they are upset at Biden. If you're in any spaces where Black US people talk to each other online, you would've seen about 70% of us, trying to convince about 20% of us, not to sit out in protest. (10% are legit Trump voters. No point trying to swing them🤷🏿♂️).