This is intentional and there are technical elements to it, though also this is to prevent accidental data loss. Resource discussion threads are a type of thread itself and thus you can't have an article (or poll or question...) be a resource discussion thread. Even if you could, the type would be changed and thus you'd lose that functionality. Depending on the forum configuration, it might not be possible to have other thread types there.
So this is very unlikely to change.