because they are different products, they have different amounts of them, there is different amount of them on the market at different prices, and the demand is different
Data Hoarder
We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
Asked and answered numerous times. Indirectly by me in another thread today. Most likely answer is supply and demand. Exos are enterprise drives and for enterprise use, bigger is definitely better and it's likely more 20TB drives are made for that market. Therefore more supply and lower price.