Literal operator template: why not string?

Once again, while replying to another question, I forgot (my fault) that literal operator templates are picked up from the set of declarations only when integer or floating literals are found.

Apart for the obvious reason that is because the standard says that, what's the technical reason (if any) for which they are not considered when dealing with string literals?

I found also a proposal for that, but still that is not an option.
What prevents them to be used for strings?