The issue is index-url extra-index-url install priority order · Issue #8606 · pypa/pip · GitHub. The maintainers aren’t fundamentally opposed to the idea, it’s just that no-one has yet come up with a proposal that addresses all of the questions involved, and handles the transition in an acceptable manner. Feel free to read the issue for a lot of discussion on “why it isn’t quite as simple as people think”.
But to cut a long story short, this can happen if someone puts together a PR and a transition plan to implement it. But if it’s just left to the pip maintainers, it will probably be quite a long time before it happens (not least because we’re all burned out on the subject).