To be honest, I feel like setup.cfg hasn’t been given a chance to prove itself useful. Just as it was made possible to use setup.cfg only (without setup.py), it’s been almost immediately taken over by pyproject.toml. However, for those who actually migrated their projects to setup.cfg, migrating once again doesn’t bring that many benefits (but e.g. loses compat with flake8).
Besides, setup.cfg’s syntax is slightly more elegant, or at least I think so :)
By the way, why do you think it is more capable? As far as I know, setup.cfg has some feature toml (intentionally) doesn’t support (not that they’re useful in this context anyway).