"If they pulled the plug today there could be as many as 40 episodes in various stages of production, you don't want to throw all that cost away, but do you really want to show 2 more seasons after the decision to cancel?"
Ahh, kind of like the Lintilla problem.
http://en.wikipedia.org/wiki/Minor_characters_from_The_Hitchhiker's_Guide_to_the_Galaxy#Lintilla