When I vote to close a question because the question belongs to another StackExchange site, I cannot afterward select any other website than metaBiology while most of the question that should be voted as belonging to another website usually belongs to philosophy, skeptics, CognitiveScience or chemistry. Why is it so?
Migration unfortunately doesn't work as well as one might expect, and that by a rather large margin. In my experience, the larger part of migration suggestions from users are not a good idea and I decline them. Very often users that are not familiar enough with the target site propose a migration, when the question is simply not appropriate for that site. Another issue is that migration is sometimes seen as a nicer alternative to closing, which leads to bad questions getting migrated and then closed.
For this reason migration paths are only enabled between sites with a significant history of successful migrations. They are also not available at all during the beta phase, both for receiving migrations and for sending them.
If you think a question would fit well on another site, and is off-topic on this one, just flag for moderator attention, select "other" and suggest a migration. Please make sure to only do this for questions of a reasonably high quality, bad questions won't get migrated.