Migration is surprisingly easy to screw up, and experiences at other SE site have shown that community migration paths can lead to a large number of bad migrations.
For that reason, only sites that have an established history of successful migrations are added to the dialog. The less related a site is, the higher is the chance that the users that vote to migrate don't properly understand it's scope and migrate an ill-fitting question.
In the meantime you can just flag for moderator attention if you want to migrate to a site that is not an official migration target.
The restricted number of migration targets is also not a technical issue, but an intentional choice. Moderators can migrate to any site via a dropdown with autocomplete: