[El Gato]

So why was the prefatory clause used there and nowhere else?

Nowhere else? There is a prefatory clause, if that's what you wish to call it, in Art. I section 8 dealing with patents and copyright (although not in those terms). Plus, as documented in The Commonplace Second Amendment many state Constitutional provisions of the time, and of times quite near to 1789, also had such clauses. In each case the explanatory, or if you insist "prefatory" clause does not limit the operative clause, which may be narrower or broader than the explanatory clause would seem to warrant.

[robertpaulsen]

"many state Constitutional provisions of the time"

I believe I said the U.S. Constitution. And my question was "why".

You posted everything but an answer.