Colossians does place emphasis on sharing Paul's epistles between the Christian communities of Colossae and Loadicea, but there is good reason to believe that Colossians was always intended to be addressed to churches other than these. John Barclay says (Colossians and Philemon, page 22) Colossians is "routinely bracketed out as deutero-Pauline and that the consensus accepts only seven letters as 'assuredly' Pauline." In other words, a clear majority of New Testament scholars believe that Paul was not really its author, and that Colossians was written in Paul's name decades after his death - and therefore not really to the Colossians in particular. This places our understanding of the epistle in an entirely new light.
Eventually, of course, all Paul's epistles were shared among all Christian churches. However, at the time of writing, Paul considered his epistles to be letters that he was writing to a specific audience and probably never considered that they could have a wider appeal.