I don't know of any offical docs or anything, but I'd imagine it's because it would be a pretty big breaking change. (before babel moved to scoped packages, it would be jarring to see `babel-core` unscoped, but `@babel/babel-plugin-thing` as scoped because it was forced. I also think there is still some roughness around "transferring" scoped packages (what happens when another user takes over a package I authored? does it change to `@OtherUser/coolThing` or does it stay under `@Klathmon/coolThing` forever? I wouldn't like that second one.)
What I would like to see is all new unscoped packages being able to be installed/required by their scoped version by default. So if I create a package `coolThing` in npm entirely unscoped, it would be installable by doing `npm -i @Klathmon/coolThing` and by doing `npm -i coolThing`
That would let many of us to use the benefits of scoped packages without the package authors having to do anything. And it would be a big step toward going completely scoped at some point in the future.