Having nothing better to do, I browsed the unanswered questions. I know that last year there was an effort to clear as many of these as possible, but of course there are still quite a few of them. Digging around further, I also found that even moderators aren't able to migrate questions over 60 days old. Some of our unanswered questions, though, are that old and (to me at least) are crying out to be migrated to, say, math.SE, tcs.SE, or, generally, someOtherMoreAppropriateSite.SE. Does anybody know or can guess the reason behind the "no migration after 60 days" rule?
Judging from the looks of it, it stems from this question on Meta Stack Exchange: Disable migration for questions older than 60 days.
If you look at the footnote, Shog9♦ mentions:
60 days is actually based on a rather unfortunate side-effect of the rep-retention changes introduced this past spring: migrated and then rejected questions older than 60 days cause the authors to suddenly gain (permanent) reputation on the destination.
I don't know what the status of the bug he alludes to is, but that seems to have been a big factor in the beginning.