Probably to cut what overhead there was in servicing users directly.

Google is still operating TLDs as "Google Registry" since they can just easy mode that part as a backend service, they just push you now to go to a domain reseller.

Because Registrars are high-effort (customers are the general public), low unit price, low-margin and Registries are low effort (customers are only Registrars), high unit price, and high margin.