There are closed questions that due to different considerations (not enough initial migration votes, question closed before an available Stack Exchange migration alternative went live, etc) were simply closed and not migrated (e.g. Should I use a 64-bit or 32-bit Linux for a high throughput LAMP server?).
Could we have a feature to propose closed questions for migration? In time, as more Stack Exchange sites go live, there will be more alternatives to migrate to, and on-topic closed questions may be migrated and used to seed those communities.