During the series and after I was frequently chatting with Bradley Thompson's significant other, Peggy Sue Davis, along with Mrs Ron, of course, and oddly enough Ron Moore's next-door neighbor, who became my Farmville neighbor (small world). Pretty early in season 2, as I recall, Bradley and David Weddle (who work as a team) got an offer to write for CSI Las Vegas, which was a major network show and a big step up from something like DS9 or Voyager, as far as Hollywood goes. So with Ron's whole-hearted support they took the offer, finishing season 2 and then starting on CSI. Peggy Sue ended up going to CSI: NY wrap parties where the Lieutenant Dan band played.
There wasn't much of a mystery, although who knows what other elements may have played into their decision, because any such decision would be complicated.