Comment by JonChesterfield

It's a nuisance to implement the thing you want to add to the standard yourself. It's easier to ship it in the language and then complain at compiler devs that they're running behind the edge of progress.

This interacts in the obvious way with refusing to correct mistakes after the fact for fear of breaking user code.

I don't believe anyone has written a paper along the lines of "let's not bother with the existing practice part anymore", it's more an emergent feature of people following local incentive structures.