I've never figured out how "migration" works, but I've repeatedly seen its existence mentioned, and I've seen migrated questions. Thus when I posted a question about the shape of the earth to math (dot) stackexchange (dot) com someone decided it belonged in history (dot) stackexchange (dot) com and somehow it got "migrated" to that site. So now I see this question posted to mathoverflow, and I wonder why it doesn't just get migrated to math (dot) stackexchange (dot) com, which seems to be where it belongs. How is that done and why would one not do it in a case like that?