This was touched on a little bit in Federated Registries?.
- There is an ecosystem cost for every new dependency table we add and we have to weigh the cost against the benefit
- There are many different requests for different extensions of the tables and they don't compose well together so we have to weigh one option against the others
- In particular, for some, just adding these tables is likely not enough and they want per-build-target dependencies (different deps for different bins). This also gets into This Development-cycle in Cargo: 1.77 | Inside Rust Blog
At least the use case you brought up to me isn't sufficient because that expresses a current local maxima of the ecosystem and I'd like to instead put my attention to helping move the ecosystem to a better place.