When the code attempts to migrate a question, one of the things it checks is the tags on the question and whether they fit on the destination site.
The question as it is on Stack Overflow is only tagged c#. This tag does not exist here. The migration process strips this tag out since it doesn't exist on the destination site, which leaves it with no valid tags. Due to this, the question is closed as off-topic instead of being migrated.
We do this to prevent a backdoor for creating tags that don't belong on the site, and to stop migrations that don't belong on the destination site in the first place.
Moderators are able to override this check when doing a migration, so you can flag these for moderator attention if something does need to be moved.
When a meta is the destination, the above tag checks are now skipped and the migration should proceed.
For all other migrations, the "one tag must exist on target site" requirement is still in effect, e.g. SO -> SU.