I looked up the press release when the first series aired two years ago and the first episode garnered 775,000 total viewers. So, while there was a drop-off, it might not be as bad as it might seem.
Of course, the mini-series aired once a week(originally Spike was going to air all five episodes at once, but they changed their mind) and on a Thursday(the most watched night on television), so that might factor into it.