This is a complex question, and probably beyond my expertise to fully answer. So let me point you to a couple of decent overview articles.

TLDR; among other issues, a lot of the seeds for varieties we used to grow actually don't exist anywhere any more.

https://thesocietypages.org/socimages/2011/07/19/loss-of-gen...

https://www.croptrust.org/our-mission/crop-diversity-why-it-...