Use the flag "In need of moderation attention" and then specify why it should be migrated and where to. It doesn't always work, depending on the length of time that the question has been posted for and some other factors, but often it does work.
The answer to "Why?" is simply because it hasn't been set up by the moderators. Generally, a specific migration "pathway" has to be agreed between sites, I think, and only then is that particular SE site manually added as an option to the migration dialog. There is a post, from a few years ago, on SE.Arduino - See Why does the migrate dialog only list Arduino Meta?.
Actually, I was mistaken, although, I am sure that I heard the above from someone, somewhere. Apparently, it is because SE 3D Printing is still in beta:
I think it is because we are "only" a Beta site. Once we graduate, I
think more migrating options become available.
As a moderator, I can migrate to other sites, and have done in some
cases. I usually ask their moderators first, and quite often they
decline to accept them. However migration does happen.
I think there is some rule that Beta sites don't get to migrate to
other sites. I'm not quite sure why such a rule exists (it would seem
more logical for it to be the other way).
See also, from SE Robotics, Closing a question - only option for another site is Robotics Meta.
Migrating to an arbitrary stack exchange site
This is only an option for ♦ moderators, so instead of voting to close
the question † you should flag it for moderator
attention ‡ with a suggestion of where it should be
On established sites, migration paths are created to allow migration
without moderator approval, but any migrations outside of those
pathways still have to be handled by a moderator. The reason for this
This specific question
This question would not be a good candidate for migration. We try not
to migrate questions which would just be closed on the target site for
another reason. In this case, it is a very broad question. It could be
edited to be a more practical, answerable questions based on actual
problems faced but
that may invalidate the existing answer, which is pretty good, given
the limitations of the question.
If neither Computer Science nor Signal Processing want this
question, I'm happy for it to remain on robitics, since there are
robotic applications of the answer to this question.
† close > off-topic because... > This question belongs on
another site in the Stack Exchange network
‡ flag > in need of moderator intervention
See also What are the limitations in Beta, in particular animuson's answer:
Beta sites don't have default migration paths (although moderators can still migrate to/from).