The rationale is, IMO, to ensure that you are actually an "expert" in the tag.
Does the top answertop answer to the "Hidden Features of C#?" question, which basically suggests Path.Combine
warrant a bronze (or indeed, I believe silver in this case!) tag badge? I think not! =)
A tag badge should indicate breadth and depth of knowledge in a subject, as well as rewarding a consistent contribution to stackoverflow in that specific subject.