It goes like this-if you're a TV exec, focused on making as much money as possible, which do you choose-keep spending $6 mil an episode on this, or make a reality show for a fifth of that, and get as good or better ratings? This is pretty much the same reason ANY show gets cancelled after 1 or 2 seasons.
Generally speaking, TV execs aren't in it for the art or the quality, they're in it for the money. The show may be brilliant, or it may be garbage, but if it's not making enough money, it's gone...and if it IS making good money, they'll try very hard to keep it going as long as they can.