My understanding was, that they rather sudenly announced that there would be no follow on season, but gave them one additional episode than was originally planned so they could write an ending. I thought that they did pretty well under the near impossible constraints they were under.
Sanctuary never even got the option of a final episode. It didn't end, it just stopped.
Oh, and Jack had the perfect line summing up the whole series. Alison said she didn;t want him to jump into that wormhole with the strong force amplifier(

) and he replied "that's what I do." EXACTLY!