Let me give a rationale for making questions space specific by quoting Atlassian documentation:
1. "In Confluence, content is organised into spaces."
If you are not putting questions to spaces the consistency of the design is broken and this will not scale well to enterprise. Of course, as with other content in confluence you should be able to search globally or within the space.
2. "You can then browse all pages with that label in a single space or across the site, or display a list of pages with that label."
Labels (which approximately equal) topics should be there to provide an extra layer of organization on top of spaces.
Note, example even Calendars get a related space, even though the calendars are going to have much lower cardinality on average compared to questions (or even topics) and the need to classify and organize them would be lower.
In the end people will use topics/labels to represent the space association, but this is semantically not clean and consistent and that matters.