Well, a casual explanation might be that because adjuncts can already move around somewhat freely, there's no need to focus them in Spec,CP. This might mean it's also possible to do so (so the complementizer could show up), or that it just isn't used like that, although it wouldn't explain why it is strictly ungrammatical. Maybe a "Last Resort" explanation? Are you sure that even with the right kind of emphatic intonation it isn't possible to get this?
In English, Do-support is said to be a Last Resort operation when tense must be spelled out without attaching to the lexical verb, but we can get an emphatic reading as in "He DID read the book." Without that emphasis it is "ungrammatical" but it is possible to generate, even as a Last Resort, if there is emphasis.