Here's the story from the Seattle Times:
Despite its long history — the harmonica was invented in 1821 — del Junco says it gets pegged as a child's toy or, as he puts it, "a fringe folk instrument." In fact, he's used to hearing, "What else do you play?" when he tells people his instrument is the harmonica.
...Originality seems as natural as breathing to this Cuban-born musician, who has been playing the harmonica since age 14. In reviews of recordings, critics have confused his sound with the saxophone or guitar, and that's not surprising. Del Junco learned "over-blowing," extending the range of the traditional 10-hole instrument, from Howard Levy, an American harmonica player. It consists of manipulating the embouchure — the way lips and tongue are applied to the instrument — to alter the pitch and bend notes.
..."People like Bob Dylan and Neil Young have given it a bad name," he quipped. "They wear a special neck brace, honk into it and play simple chord melodies. Jazz and blues have been a much more underground music, so a lot of the public associate the harmonica with that."
As far as causes go, this one is pretty weak. But I guess I should just leave a tender moment alone.