According to what I've ready, originally, the Morn costume didn't allow for the actor to speak. Eventually they fixed that problem, but by that time, it had become an in-joke that Morn was supposed to be a chatterbox yet we never heard him speak on camera, so they kept it that way. Fun fact about the episode "Who Mourns for Morn?": when everyone thinks Morn is dead, Quark urges the bar patrons to keep Morn's chair warm for him at the memorial service. He has a Bajoran man sit in the chair and everyone applauds. The Bajoran dude is actually the guy who plays Morn (without the Morn suit/make-up, of course).
, you forget, Morn ran a shipping business too! He didn't plan on touching the heist money until after the statute of limitations had expired. Apparently between all the sitting around at Quark's he managed to find some time to run his business as well.