Why when closing because a migration is needed, if I do:
"off-topic because..."
and then
"This question belongs on another site in the Stack Exchange network"
The only destination available is Earth Sciences Meta:
Why is that? How I can suggest migrations to other sites?
I think I've seen other sites in the network listed there.
Migration paths to other sites are something that are very limited in a few ways.
First, sites that are in beta, like Earth Sciences, don't generally have the option to have migration paths at all. We allow migration between the main and meta sites by default because it's quite common for the two to get mixed up but if you feel a question needs to be moved to a different Stack Exchange site, that's going to require the work of a moderator.
Even on graduated sites there needs to be strong support from the community to create a migration path and we restrict each site to having only five at the most (plus Meta).
As I alluded to earlier, your moderators have the ability to migrate a post to any site they wish. If a question is completely off topic here - say about pie crust instead of the Earth's crust - you can flag the post for your moderators' attention and explain that the post should be moved to Seasoned Advice. If they agree with you, they may complete the migration.
There are a few considerations with migrations and I generally recommend that you close the off-topic question and encourage the OP to ask the question on the other site directly rather than migrating it. It's often faster, since you don't need to wait for a moderator, and there are some benefits for the asker.
This and much more is explained in detail in the Migration FAQ on Meta Stack Exchange.
Hi! I'm Catija, one of your Community Managers.
