Rearchitecting the storage layer takes time. A storage manager API didn't even exist until fairly recently, maybe 14. That API needs to undergo changes to account for things that Oriole is trying to do. Postgres is not developed by a team. It's a community effort, and people work on what they want to work on. If you're interested in a previous attempt to change the storage layer, you can learn about what happened with zheap[0].

[0]: https://pgpedia.info/z/zheap.html