As a Linux system administrator/DevOps/whatever, I use Stack Overflow all the time in my work. I also (to a much lesser extent) use Server Fault and the Unix & Linux Stack Exchange. The reason I don't use the latter two more often is that most of the questions I want answered were answered years ago before those new branches emerged, and not every question that could/should be moved over has been.
I'm not really surprised, since migrating a question has to be done by the moderators. That would be an insane amount of work. What if we distributed that workload a bit?
I propose, besides being able to upvote/downvote a question, you could also vote to have it migrated to another Stack Exchange site. Then the moderators could easily find the questions that most need to be migrated by their vote-count, or we could even have them auto-migrate if the voting ratio hits some threshold.