It's really bugging me. There's a question that I believe belongs on a stackExchange site which is not included in the list below:
Who has created that list anyway? Why can't I see any Search field?
It is created and maintained by the devs, with collaboration from the community and mods. This is because mods typically ask the mods on sites where we think a question may be on topic, and move accordingly.
Not everyone reads the help center for the sites they are trying to send a question to, so we only maintain a small list for regular users to migrate to.
If you feel it belongs on a site not listed, feel free to Flag for Moderator attention (after reading the destination site's Help Center), and they can make the call. Option two (and usually faster) is to come into the Ask a SU Mod chat room, and ask there.
This was brought up on Meta Stack Exchange a few years back. The main reasoning for not adding more was:
Adding even more migration paths to this existing close list would clutter the UI. The idea is to show the most frequent migration paths so the users to can migrate most questions.
Along with a variety of other reasons that were brought up. Here's a few more:
Unless you are very familiar with the community that will be receiving the question, you should not be migrating it there. Moderators can get in touch with other moderators on the receiving site when in doubt.
Questions not in the given paths to migrate should not be migrated unless there is a compelling reason to do so. The end result of a migration is ensuring that a question gets more high quality answers faster than it would otherwise.
Another reason for the limited list is that we do not migrate questions to Stack Exchange sites that are in Beta. So, only the well-established related SE sites are listed.