Sure, but what if the language I'm using is not high on your priority list? Isn't it easier to add a high level rest api?
If you look at the work required to write a simplified read-only client using the AWS SDK for KMS and DynamoDB, while perhaps a bit more work than integrating with a REST API, it is not that far off, which is why we are considering that approach. That would of course not help if you are using a language that does not have an AWS SDK.
I could definitely envision people deciding against using your key store when comparing to something like Vault based just on the fact that you limit extensibility with no API.