There have been a number of Meta posts confused as to Why can't I migrate this particular question? There's no feedback indicating why you don't get the "belongs on" menu when selecting the option, so I'm not surprised:
A section should pop-out below the Off Topic block explaining why you can only vote off topic, not to migrate, when the 60 day limit has been reached for a post. This should alleviate a lot of the confusion, and you could toss in a link to the official meta post about the feature change to explain the reasoning too.