Not should be migrated. Can be migrated.
A lot of the questions you'll find on Stack Overflow that could be asked on another site are still on-topic on Stack Overflow and may have been posted before these other sites even existed.
There's not much point in just moving things around for the sake of moving things around. It's confusing for the askers, the answerers, and there are often nuances on the more specialized sites that make Stack Overflow questions poor fits for them. (For example, DBA.SE is into more complex questions and not just anything with SQL in it.)
In addition to all that, questions that are more than 60 days old cannot be migrated at all.
Far as your update goes... it depends a bit on the question. Stack Overflow is for programming
problems. For Drupal this would be things like creating modules or modifying themes. (I think. I'm not actually familiar with Drupal.)
General Drupal usage questions such as module suggestions, troubleshooting, etc. would probably fare better over on Drupal.SE. Check out their FAQ for details on what's on-topic.
Like I said, I'm not very familiar with Drupal, but based on what I know, I think most development questions could land on either site.