This was tried many years ago. At that time, Promises/A+ was not finalized, and the community could not agree on which Promise specification was best, or even if one was needed at all.

Callbacks are lightest-weight re: CPU & memory overhead, so it was decided that core APIs should implement that, and developers can override using promisify (via e.g. Bluebird or the new `util.promisify()`) as they need. But putting that kind of assumption in core could lead to significant pain.