Questions are only migrated when there is a problem, specifically they need to be off-topic on the site on which they are posted.

Computer software and hardware are on-topic on SU - including Ubuntu, Linux, Unix, Mac, etc - so questions that are valid for [Ask Ubuntu][3], [Unix & Linux][2] or [Ask Different][1] are usually valid here as well (the obvious exception being iPhone, etc questions which are OK on AD, but not here).

The sites have different community spirits and different community goals, so it's important that if a user attempts to interact with one of these communities that we don't push them away. So if someone has made the effort to ask a valid question on SU, we will not automatically shunt it to another site.

Migrating a question can be damaging to the user experience of the owner and greatly increases the chance of the question being abandoned.

Finally, in the cases when we do migrate to these sites it's usually because the question hasn't received much attention locally and - more importantly - because the asker has requested that it be moved.

[1]: http://apple.stackexchange.com/
[2]: http://unix.stackexchange.com/
[3]: http://askubuntu.com/