This isn't new, migration of questions older than 60 days has been disabled since October 2012.
Let's assume for a minute this wasn't the case, and we migrated the question. It would carry with it a +10 answer, which - for Politics.SE's size and age - looks pretty authoritative. However, none of those upvotes have been given by Politics members.
That's not good. People browsing Politics expect answers to be vetted by politics experts, not by historians and history buffs. Don't get me wrong, ihtkwot's answer happens to be a good one, it simply hasn't been vetted by the appropriate crowd.
There are a couple of things you can do:
- If you really want the question on Politics, you could simply re-ask it there. Ideally ihtkwot would also consider re-posting his answer, and then we would delete the History version.
- If you think there are a lot of closed questions on History that should be migrated to Politics, then you should make a list. When you're done, you could invite the Politics crowd to check out the list, decide which questions we want, and ask Stack Exchange to migrate them manually.