It's not always beneficial to have other sites show up in the list. A prime example is Programmers, which used to be on the list for migration from StackOverflow but no longer is. As I understand it, lots of people on SO would mark questions for migration to Programmers because they were off topic for SO but still seemed related to programming. Understandable, but Programmers isn't just a catch-all for everything that contains the word "computer" and isn't appropriate for SO, and the volume of incorrect migrations was becoming a problem, so they removed Programmers from the list.
You can still get questions migrated to Programmers, of course; there's just not an automatic way to suggest that. Instead, flag the question and add a message to the moderator suggesting the migration. It'd probably make things a little easier on the mods if you add a quick explanation of why the question is appropriate for Programmers. Same goes for other sites like Code Review.