Migrations are often a [point of contention between communities](http://blog.stackoverflow.com/2012/03/respect-the-community-your-own-and-others/), so we are very conservative when it comes to adding default migration paths. In essence, a site would need to show a history of successful migrations before a new default can be added. (Yes, this is a bit of a Catch-22 since you'll need a moderator's help to migrate to a site not on the default migration path.) [In the past 90 days](http://mathoverflow.net/tools/posts/migrated/stats), there have been 3 migrations to Academia and one of those was rejected. Compared to the current defaults (Mathematics—140 migrations; 10 rejected and Statistics—10 migrations; 1 rejected) there's not a lot of evidence for this migration path. So for the moment, I won't be adding a migration path. But we can look again if there are more migrations to Academia in the future.