This was discussed in More Options When Flagging for Migration. Quoting Jeff:
There are only 5 blessed migration slots, of which 1 is dedicated to
meta.
This is a "Don't Make Me Think" barrier; we don't want to present
closers with a list of 12 different destinations and make them suss
out which one is correct. There should be 4 most frequently correct
destinations + meta. If you believe a pet site deserves a place in
that list, prove it with data of existing closed questions, not blue
sky "what if there were.." imagineering
If you feel strongly a question belongs on a site that isn't listed,
then there is a way to do this -- flag them for moderator attention
instead!
Can you make a case that these Unix questions are more prevalent than
one of the other 4, and should displace it in the list? Do you have
data to support this, in the form of a giant list of closed questions
that belonged on {x} site?
(I am also disinclined to randomly migrate questions in the network
unless they are of good quality, and strong fits to the target
site.)
Personally, I flag them as "other" and include where I think they belong in the message to the moderators.