As you can see from the screenshot below, there is no option to move this question to Stack Overflow. Is that a bug, or is there some reason this option is unavailable?
This is not a bug, the question is older than 60 days so the migration option is disabled.
See this Meta Stack Overflow Post:
questions older than 60 days should not be eligible for migration, either by normal user close-votes or by moderators. In extreme cases, we can perhaps provide an alternate means of moving old questions, but under normal circumstances these should be done quickly or not at all.