IMHO this has two reasons:
software developers (Stack Overflow) and system administrators (SuperUser) might have very special needs - and the required program simply does not exist
there is no migration path (yet). We should make sure that SR appears here on SO:
I personally would flag questions like that, which in turn would train SO users. I didn't ask for moderator migration until now, instead vote for off-topic. This should no longer be needed since SR is out of beta.
This will not immediately reduce the number of open questions, in fact we might get a lot of crappy new questions here. But over time, Stack Overflow and SuperUser users will find this place useful, ask better questions and answer other's questions.