Good evening,
I‘m pretty sure, because there are a lot of relations in that datasource and not suggested in the list of sources… even though the results will end up in an array or „compareable“ and not in a column->rows scene.
Any Multiple Relation or Query should show up as a source for a choice component as long as the screen is connected to the table that contains that Query or Relation. Make sure you are looking for the Relation directly in the source list, and not looking for it through a table in that source list.
Yah I just tried and seems like you can only point to tables using a choice/tag option. I think you would have to resort to a custom collection and use a choice component there.