Just part of his mystique. "Morn" is an anagram of "Norm", an oblique reference to Cheers. Also, being able to ingest latinum is a good skill.
As a matter of fact, the mouth of the Morn mask was capible of moving and allowed the actor to speak, and the writers repeatedly toyed with the idea of giving Morn lines, at one point he was even going to get the last line of the series, but they decided against it, as it had become a bit of a running gag on the show. We do know that Morn is capible of speaking, as other characters have said he has quite the reputation for being talkative.